xfs: whiteouts release intents that are not in the AIL

xfs: whiteouts release intents that are not in the AIL

When we release an intent that a whiteout applies to, it will not
have been committed to the journal and so won't be in the AIL. Hence
when we drop the last reference to the intent, we do not want to try
to remove it from the AIL as that will trigger a filesystem
shutdown. Hence make the removal of intents from the AIL conditional
on them actually being in the AIL so we do the correct thing.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>