block/drbd: Use the enum req_op and blk_opf_t types

Improve static type checking by using the enum req_op type for variables
that represent a request operation and the new blk_opf_t type for
variables that represent request flags.

Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Cc: Lars Ellenberg <lars.ellenberg@linbit.com>
Cc: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-14-bvanassche@acm.org


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