dm: abstract dm_in_flight function

This patch adds md_in_flight() to get the number of in_flight I/Os.
No functional change.

This patch is a preparation for a later patch in this series, which
changes I/O counter to md->pending from q->in_flight in request-based dm.
Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -452,6 +452,12 @@ static void free_bio_info(struct dm_rq_clone_bio_info *info)
 	mempool_free(info, info->tio->md->io_pool);
 }
 
+static int md_in_flight(struct mapped_device *md)
+{
+	return atomic_read(&md->pending[READ]) +
+	       atomic_read(&md->pending[WRITE]);
+}
+
 static void start_io_acct(struct dm_io *io)
 {
 	struct mapped_device *md = io->md;
@@ -2100,8 +2106,7 @@ static int dm_wait_for_completion(struct mapped_device *md, int interruptible)
 				break;
 			}
 			spin_unlock_irqrestore(q->queue_lock, flags);
-		} else if (!atomic_read(&md->pending[0]) &&
-					!atomic_read(&md->pending[1]))
+		} else if (!md_in_flight(md))
 			break;
 
 		if (interruptible == TASK_INTERRUPTIBLE &&