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

Cc: Mike Christie <michael.christie@oracle.com>
Cc: Bodo Stroesser <bostroesser@gmail.com>
Cc: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-45-bvanassche@acm.orgSigned-off-by: NJens Axboe <axboe@kernel.dk>