block: account merge of two requests

Also rename blk_account_io_merge() into blk_account_io_merge_request() to
distinguish it from merging request and bio.

[hch: rebased]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-merge.c

@@ -669,18 +669,16 @@ void blk_rq_set_mixed_merge(struct request *rq)
 	rq->rq_flags |= RQF_MIXED_MERGE;
 }
 
-static void blk_account_io_merge(struct request *req)
+static void blk_account_io_merge_request(struct request *req)
 {
 	if (blk_do_io_stat(req)) {
-		struct hd_struct *part;
-
 		part_stat_lock();
-		part = req->part;
-
-		hd_struct_put(part);
+		part_stat_inc(req->part, merges[op_stat_group(req_op(req))]);
+		hd_struct_put(req->part);
 		part_stat_unlock();
 	}
 }
+
 /*
  * Two cases of handling DISCARD merge:
  * If max_discard_segments > 1, the driver takes every bio
@@ -792,7 +790,7 @@ static struct request *attempt_merge(struct request_queue *q,
 	/*
 	 * 'next' is going away, so update stats accordingly
 	 */
-	blk_account_io_merge(next);
+	blk_account_io_merge_request(next);
 
 	/*
 	 * ownership of bio passed from next to req, return 'next' for