audit: Guarantee forward progress of chunk untagging

When removing chunk from a tree, we do shrink the chunk. This can fail
for various reasons (due to races, ENOMEM, etc.) and in some cases we
just bail from untag_chunk() relying on someone else to cleanup.
Although this currently works, later we will need to add new failure
situation which would break. Also this simplifies the code and will
allow us to make locking around untag_chunk() less awkward.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>