blk-mq: move more request initialization to blk_mq_rq_ctx_init

blk-mq: move more request initialization to blk_mq_rq_ctx_init

Don't split request initialization between __blk_mq_alloc_request and
blk_mq_rq_ctx_init.  Also remove the op argument as it can be derived
from the blk_mq_alloc_data structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>