mpage: pass the operation to bio_alloc

Refactor the mpage read/write page code to pass the op to bio_alloc
instead of setting it just before the submission.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220222154634.597067-2-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>