blk-mq: merge blk_mq_insert_request and blk_mq_run_request

It's almost identical to blk_mq_insert_request, so fold the two into one
slightly more generic function by making the flush special case a bit
smarted.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/blk-exec.c
Simple merge

diff --cc block/blk-flush.c
Simple merge

diff --cc block/blk-mq.c
Simple merge

diff --cc block/blk-mq.h
Simple merge

diff --cc include/linux/blk-mq.h
Simple merge