xfs: fix NULL pointer dereference in xfs_filestream_lookup_ag()

If xfs_filestream_get_parent() fails, we have a null pip,
goto out, and attempt to IRELE(NULL).  This causes a null
pointer dereference and BUG().

Fix this by directly returning NULLAGNUMBER in this case.
Reported-by: NAdrien Nader <adrien@notk.org>
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/xfs/xfs_filestream.c

@@ -322,7 +322,7 @@ xfs_filestream_lookup_ag(
 
 	pip = xfs_filestream_get_parent(ip);
 	if (!pip)
-		goto out;
+		return NULLAGNUMBER;
 
 	mru = xfs_mru_cache_lookup(mp->m_filestream, pip->i_ino);
 	if (mru) {