xfs: refactor xfs_iread_extents to use xfs_btree_visit_blocks

xfs_iread_extents open-codes everything in xfs_btree_visit_blocks, so
refactor the btree helper to be able to iterate only the records on
level 0, then port iread_extents to use it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/scrub/bitmap.c

@@ -294,5 +294,6 @@ xfs_bitmap_set_btblocks(
 	struct xfs_bitmap	*bitmap,
 	struct xfs_btree_cur	*cur)
 {
-	return xfs_btree_visit_blocks(cur, xfs_bitmap_collect_btblock, bitmap);
+	return xfs_btree_visit_blocks(cur, xfs_bitmap_collect_btblock,
+			XFS_BTREE_VISIT_ALL, bitmap);
 }