[XFS] fix compile of xfs_btree_readahead_lblock on m68k

Change the left/right variables to the proper always 64bit xfs_dfsbo_t
type because otherwise compilation fails for Geert on m68k without
CONFIG_LBD:

| fs/xfs/xfs_btree.c: In function 'xfs_btree_readahead_lblock':
| fs/xfs/xfs_btree.c:736: warning: comparison is always true due to limited range of data type
| fs/xfs/xfs_btree.c:741: warning: comparison is always true due to limited range of data type

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

fs/xfs/xfs_btree.c

@@ -730,8 +730,8 @@ xfs_btree_readahead_lblock(
 	struct xfs_btree_block	*block)
 {
 	int			rval = 0;
-	xfs_fsblock_t		left = be64_to_cpu(block->bb_u.l.bb_leftsib);
-	xfs_fsblock_t		right = be64_to_cpu(block->bb_u.l.bb_rightsib);
+	xfs_dfsbno_t		left = be64_to_cpu(block->bb_u.l.bb_leftsib);
+	xfs_dfsbno_t		right = be64_to_cpu(block->bb_u.l.bb_rightsib);
 
 	if ((lr & XFS_BTCUR_LEFTRA) && left != NULLDFSBNO) {
 		xfs_btree_reada_bufl(cur->bc_mp, left, 1);