Rename follow-up and gcc warning fix - #14813

../src/core/selinux-access.c: In function ‘mac_selinux_generic_access_check’:
../src/basic/log.h:223:27: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
../src/core/selinux-access.c:235:85: note: format string is defined here
  235 |                         log_warning_errno(errno, "SELinux getcon_raw failed (tclass=%s perm=%s): %m", tclass, permission);
      |                                                                                     ^~

I wonder why nobody ever noticed this.

Fixes systemd#14691 (other issues listed in that ticket have already been fixed).

Instead of setting the bus error structure and then freeing it, let's only set
it if used. If we will ignore the selinux denial, say ", ignore" to make this
clear. Also, use _cleanup_ to avoid gotos.