xen-blkback: Use the enum req_op and blk_opf_t types

Improve static type checking by using the enum req_op type for request
operations and the new blk_opf_t type for request flags.

Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-18-bvanassche@acm.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>