md/bcache: Combine two uuid_io() arguments

Improve uniformity in the kernel of handling of request operation and
flags by passing these as a single argument.

Cc: Coly Li <colyli@suse.de>
Cc: Mingzhe Zou <mingzhe.zou@easystack.cn>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-33-bvanassche@acm.org


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