dm/core: Combine request operation type and flags

Improve kernel code uniformity by combining the request operation type and
flags into a single variable. Change 'int rw' into 'enum req_op op' because
the name 'op' is what is used in the block layer to hold a request type.
Use the blk_opf_t and enum req_op types where appropriate to improve static
type checking.

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/20220714180729.1065367-24-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>