xfs: use ->t_firstblock in attrfork add

Note that this codepath is a user of struct xfs_da_args. Switch it
over to ->t_firstblock in preparation to remove
xfs_da_args.firstblock.

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>