dm: simplify start of block stats accounting for bio-based

No apparent need to generic_start_io_acct() until before the IO is ready
for submission.  start_io_acct() is the proper place to do this
accounting -- it is also where DM accounts for pending IO and, if
enabled, starts dm-stats accounting.

Replace start_io_acct()'s part_round_stats() with generic_start_io_acct().
This eliminates needing to take part_stat_lock() multiple times when
starting an IO on bio-based devices.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -599,16 +599,14 @@ static void start_io_acct(struct dm_io *io)
 {
 	struct mapped_device *md = io->md;
 	struct bio *bio = io->orig_bio;
-	int cpu;
 	int rw = bio_data_dir(bio);
 
 	io->start_time = jiffies;
 
-	cpu = part_stat_lock();
-	part_round_stats(md->queue, cpu, &dm_disk(md)->part0);
-	part_stat_unlock();
+	generic_start_io_acct(md->queue, rw, bio_sectors(bio), &dm_disk(md)->part0);
+
 	atomic_set(&dm_disk(md)->part0.in_flight[rw],
-		atomic_inc_return(&md->pending[rw]));
+		   atomic_inc_return(&md->pending[rw]));
 
 	if (unlikely(dm_stats_used(&md->stats)))
 		dm_stats_account_io(&md->stats, bio_data_dir(bio),
@@ -1556,15 +1554,12 @@ static void __split_and_process_bio(struct mapped_device *md,
  */
 static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
 {
-	int rw = bio_data_dir(bio);
 	struct mapped_device *md = q->queuedata;
 	int srcu_idx;
 	struct dm_table *map;
 
 	map = dm_get_live_table(md, &srcu_idx);
 
-	generic_start_io_acct(q, rw, bio_sectors(bio), &dm_disk(md)->part0);
-
 	/* if we're suspended, we have to queue this io for later */
 	if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
 		dm_put_live_table(md, srcu_idx);