[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>