blk-mq: fix initializing request's start time

blk_rq_init() is called in req's complete handler to initialize
the request, so the members of start_time and start_time_ns might
become inaccurate when it is allocated in future.

The patch initializes the two members in blk_mq_rq_ctx_init() to
fix the problem.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6914f9b..473ce40 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -190,6 +190,8 @@
 
 	rq->mq_ctx = ctx;
 	rq->cmd_flags = rw_flags;
+	rq->start_time = jiffies;
+	set_start_time_ns(rq);
 	ctx->rq_dispatched[rw_is_sync(rw_flags)]++;
 }