xfs: remove xfs_trans_get_block_res

Just use the t_blk_res field directly instead of obsfucating the reference
by a macro.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>