tmpfiles: don't relabel files in dry run mode

tmpfiles: don't relabel files in dry run mode

tmpfiles attempts to correct the label of a file during various actions
via the function fd_set_perms().  Currently, said function generally
respects the dry-run mode.  However, it attempts to fix the label of a
given file regardless of the state of said dry-run mode.

This causes problems, because a user could attempt to run tmpfiles with
elevated permissions and dry run enabled, expecting the tool to not
modify their system.  Instead, tmpfiles would falsely relabel a file,
modifying their system.

This commit explicitly checks for when dry-run is enabled and skips the
file relabelling process.  Furthermore, I added logging for both cases.
I found helpful during debugging.  That said, I don't think it's
necessary to use the level LOG_INFO on the dry-run path, as it would
always produce an info log.