block: add a rq_dma_dir helper

In a lot of places we want to know the DMA direction for a given
struct request.  Add a little helper to make it a littler easier.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>