xfs: use ->t_firstblock in dir ops

Callers of the xfs_dir_*() functions currently pass an on-stack
firstblock variable. While the dirops infrastructure carries a
pointer to this variable, it never rolls the transaction and so it
is safe to use ->t_firstblock instead.

Fix up the various xfs_dir_*() callers to use ->t_firstblock. Also
remove the unnecessary parameter for xfs_cross_rename().

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>