blk-mq: update hardware and software queues for sleeping alloc

If we end up sleeping due to running out of requests, we should
update the hardware and software queues in the map ctx structure.
Otherwise we could end up having rq->mq_ctx point to the pre-sleep
context, and risk corrupting ctx->rq_list since we'll be
grabbing the wrong lock when inserting the request.

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Chris Mason <clm@fb.com>
Tested-by: Chris Mason <clm@fb.com>
Fixes: 63581af3

 ("blk-mq: remove non-blocking pass in blk_mq_map_request")
Signed-off-by: Jens Axboe <axboe@fb.com>