blk-mq: allocate flush_rq in blk_mq_init_flush()

It is reasonable to allocate flush req in blk_mq_init_flush().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

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

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

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