xfs: fix btree scrub checking with regards to root-in-inode

In xchk_btree_check_owner, we can be passed a null buffer pointer.  This
should only happen for the root of a root-in-inode btree type, but we
should program defensively in case the btree cursor state ever gets
screwed up and we get a null buffer anyway.

Coverity-id: 1438713
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/scrub/btree.c

@@ -415,8 +415,17 @@ xchk_btree_check_owner(
 	struct xfs_btree_cur	*cur = bs->cur;
 	struct check_owner	*co;
 
-	if ((cur->bc_flags & XFS_BTREE_ROOT_IN_INODE) && bp == NULL)
+	/*
+	 * In theory, xfs_btree_get_block should only give us a null buffer
+	 * pointer for the root of a root-in-inode btree type, but we need
+	 * to check defensively here in case the cursor state is also screwed
+	 * up.
+	 */
+	if (bp == NULL) {
+		if (!(cur->bc_flags & XFS_BTREE_ROOT_IN_INODE))
+			xchk_btree_set_corrupt(bs->sc, bs->cur, level);
 		return 0;
+	}
 
 	/*
 	 * We want to cross-reference each btree block with the bnobt