xfs: shut up -Wuninitialized in xfsaild_push

-Wuninitialized complains about @target in xfsaild_push being
uninitialized in the case where the waitqueue is active but there is no
last item in the AIL to wait for.  I /think/ it should never be the case
that the subsequent xfs_trans_ail_cursor_first returns a log item and
hence we'll never end up at XFS_LSN_CMP, but let's make this explicit.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/xfs_trans_ail.c

@@ -422,7 +422,7 @@ xfsaild_push(
 	struct xfs_ail_cursor	cur;
 	struct xfs_log_item	*lip;
 	xfs_lsn_t		lsn;
-	xfs_lsn_t		target;
+	xfs_lsn_t		target = NULLCOMMITLSN;
 	long			tout;
 	int			stuck = 0;
 	int			flushing = 0;
@@ -472,6 +472,8 @@ xfsaild_push(
 
 	XFS_STATS_INC(mp, xs_push_ail);
 
+	ASSERT(target != NULLCOMMITLSN);
+
 	lsn = lip->li_lsn;
 	while ((XFS_LSN_CMP(lip->li_lsn, target) <= 0)) {
 		int	lock_result;