xfs: remove firstblock param from xfs dir ops

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>