dm mirror log: Use the new blk_opf_t type

Improve static type checking by using the new blk_opf_t type for a function
argument that represents a request operation type.

Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-28-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>