md/raid1: Use the new blk_opf_t type

Improve static type checking by using the new blk_opf_t type for
variables that represent request flags.

Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-35-bvanassche@acm.org


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