block: initialize flush request with WRITE_FLUSH instead of REQ_FLUSH

init_flush_request() only set REQ_FLUSH when initializing flush
requests making them READ requests.  Use WRITE_FLUSH instead.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/blk-flush.c

@@ -77,7 +77,7 @@ static void post_flush_end_io(struct request *rq, int error)
 static void init_flush_request(struct request *rq, struct gendisk *disk)
 {
 	rq->cmd_type = REQ_TYPE_FS;
-	rq->cmd_flags = REQ_FLUSH;
+	rq->cmd_flags = WRITE_FLUSH;
 	rq->rq_disk = disk;
 }