block: add rq_flags to struct blk_mq_alloc_data

There's a hole here we can use, and it's faster to set this earlier
rather than need to check q->elevator multiple times.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20211019153300.623322-2-axboe@kernel.dk


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