fix(kubernetes): log kubevirt-csi temp NFS mount dir removal failure - #4562

…out (#4584)

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## What this PR does

The kubevirt-csi node plugin now unmounts its temporary NFS mount with a
timeout, and retries with `umount -f` when the timeout runs out.

For an NFS publish the plugin mounts the export root into an
`nfs-init-*` temp dir to create `/data`. It unmounted that dir in a
defer with a plain `umount`, which has no deadline. Before this change,
if the NFS server was gone by then, the publish could hang on that
`umount` forever and the temp mount stayed.

The force-unmount helper in `k8s.io/mount-utils` doesn't work for this
case. `UnmountWithForce` retries with `-f` only after its first `umount`
returns. That `umount` forks the `umount.nfs` helper. When the timeout
kills `umount`, the helper stays stuck on the dead server and keeps the
output pipe open, and `CombinedOutput` waits for that pipe, so the `-f`
retry never runs. The plugin now runs `umount` itself with `WaitDelay`
set, so the pipe is closed a second after the kill, and then runs
`umount -f` under its own timeout. A "not mounted" answer counts as
done, same as in mount-utils. `umount2` detaches the mount before it
blocks on tearing down the superblock, so the `-f` may find nothing
left.

Some umounts can't be killed at all, for example one stuck in
uninterruptible sleep. So the whole cleanup also waits at most 30s, well
inside the 2m deadline kubelet sets for NodePublishVolume (`csiTimeout`
in `pkg/volume/csi/csi_plugin.go`). Once the budget runs out the publish
goes on, and the unmount keeps running in the background and logs its
result when it ends. The temp dir is removed by that background unmount
once it succeeds, so an unmount that finishes late still cleans up after
itself. A mounter other than the exec-based one keeps its plain
`Unmount`, under the same budget.

The temp mount now uses `nosharecache`. A forced unmount runs
`nfs_umount_begin`, which kills every RPC in flight on the superblock,
and NFS shares a superblock between mounts of the same export with the
same options. Without this option, `-f` on the temp mount could fail I/O
on the pod mount this same publish creates, and on other pods on the
node that mount the same volume. The cost is a separate cache: pods that
already have `/data` mounted may not see what the migration moves in
until their attribute cache expires. The migration is a one-time
backfill, so I'm fine with that.

The leak only lives until the container restarts, because `/tmp` in the
node plugin container has no hostPath and the mounts go away with the
container's mount namespace. So this change stops leaked mounts from
piling up inside one container. For the same reason a sweep of
`nfs-init-*` dirs at startup would never find anything, so I did not add
one.

Two cases stay as they were. An `umount` that fails fast, for example
with `target is busy`, gets no force retry, is logged, and the dir stays
because `rmdir` refuses a mount point. And since the temp mount is a
hard mount, a server that disappears before the unmount still hangs the
mount and migration calls earlier in this path. That second case is
#4581.

The tests run the real exec-based mounter against a fake `umount` on
`PATH`. The fake covers a plain unmount that succeeds, one that fails
fast and gets no `-f`, and one that hangs while an orphaned child holds
its output pipe. The last one has to end in `umount -f` inside the
budget. A "not mounted" from `-f` counts as done, and any other `-f`
failure is reported and keeps the dir. Fake mounters cover the
background path: giving up on a stalled unmount, removing the dir after
a late unmount, and logging the late result. Other tests check
`nosharecache` on the temp mount and the temp dir after a failed mount.
I haven't reproduced this against a real NFS server that went away, so
it is not checked whether `umount -f` in the almalinux image gets as far
as `umount2(MNT_FORCE)` there. The tests are in a new file, so this PR
does not overlap with #4562.

Closes #4561

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No UI change.

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### Release note

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```release-note
fix(kubernetes): the kubevirt-csi node plugin no longer hangs an NFS volume publish forever when unmounting its temporary mount after the NFS server went away; the unmount now times out and is retried with force
```




<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Temporary NFS mounts use isolated caching to reduce interference with
pod mounts.
* Cleanup waits up to 30 seconds for unmounting. If the limit is
reached, the publish operation returns an error while unmounting
continues in the background.
* Temporary directories are removed after a successful unmount or a
failed mount, and retained if unmounting fails.
* When a standard unmount times out, cleanup retries with a forced
unmount when available.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->