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>