ext4: pass the operation to bio_alloc

Refactor the readpage code to pass the op to bio_alloc instead of setting
it just before the submission.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20220222154634.597067-3-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>