xfs: fix imprecise logic in xchk_btree_check_block_owner

A reviewer was confused by the init_sa logic in this function.  Upon
checking the logic, I discovered that the code is imprecise.  What we
want to do here is check that there is an ownership record in the rmap
btree for the AG that contains a btree block.

For an inode-rooted btree (e.g. the bmbt) the per-AG btree cursors have
not been initialized because inode btrees can span multiple AGs.
Therefore, we must initialize the per-AG btree cursors in sc->sa before
proceeding.  That is what init_sa controls, and hence the logic should
be gated on XFS_BTREE_ROOT_IN_INODE, not XFS_BTREE_LONG_PTRS.

In practice, ROOT_IN_INODE and LONG_PTRS are coincident so this hasn't
mattered.  However, we're about to refactor both of those flags into
separate btree_ops fields so we want this the logic to make sense
afterwards.

Fixes: 858333dc ("xfs: check btree block ownership with bnobt/rmapbt when scrubbing btree")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/scrub/btree.c

@@ -385,7 +385,12 @@ xchk_btree_check_block_owner(
 	agno = xfs_daddr_to_agno(bs->cur->bc_mp, daddr);
 	agbno = xfs_daddr_to_agbno(bs->cur->bc_mp, daddr);
 
-	init_sa = bs->cur->bc_flags & XFS_BTREE_LONG_PTRS;
+	/*
+	 * If the btree being examined is not itself a per-AG btree, initialize
+	 * sc->sa so that we can check for the presence of an ownership record
+	 * in the rmap btree for the AG containing the block.
+	 */
+	init_sa = bs->cur->bc_flags & XFS_BTREE_ROOT_IN_INODE;
 	if (init_sa) {
 		error = xchk_ag_init_existing(bs->sc, agno, &bs->sc->sa);
 		if (!xchk_btree_xref_process_error(bs->sc, bs->cur,