Improve static type checking by using the new blk_opf_t type for variables
that represent a request operation combined with request flags. Rename
those variables from 'op' into 'opf'.

Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-39-bvanassche@acm.orgSigned-off-by: Jens Axboe <axboe@kernel.dk>