xfs: lock the AIL before removing the buffer item

Regression introduced by commit 46f9d2eb ("xfs: aborted buf items can
be in the AIL") which fails to lock the AIL before removing the
item. Spinlock debugging throws a warning about this.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_buf_item.c

@@ -628,6 +628,7 @@ xfs_buf_item_unlock(
 		else if (aborted) {
 			ASSERT(XFS_FORCED_SHUTDOWN(lip->li_mountp));
 			if (lip->li_flags & XFS_LI_IN_AIL) {
+				spin_lock(&lip->li_ailp->xa_lock);
 				xfs_trans_ail_delete(lip->li_ailp, lip,
 						     SHUTDOWN_LOG_IO_ERROR);
 			}