xfs: remove firstblock param from xfs dir ops

xfs: remove firstblock param from xfs dir ops

Source kernel commit: 381eee69f862d38bef468e91517e37fc53f60885

All callers of the xfs_dir_*() functions pass ->t_firstblock as the
firstblock parameter. Drop the parameter and access ->t_firstblock
directly.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>