dm thin: avoid metadata commit if a pool's thin devices haven't changed

Commit 905e51b ("dm thin: commit outstanding data every second")
introduced a periodic commit.  This commit occurs regardless of whether
any thin devices have made changes.

Fix the periodic commit to check if any of a pool's thin devices have
changed using dm_pool_changed_this_transaction().

Reported-by: Alexander Larsson <alexl@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>
Cc: stable@vger.kernel.org
diff --git a/drivers/md/dm-thin.c b/drivers/md/dm-thin.c
index faaf944..d501e43 100644
--- a/drivers/md/dm-thin.c
+++ b/drivers/md/dm-thin.c
@@ -1357,7 +1357,8 @@
 	bio_list_init(&pool->deferred_flush_bios);
 	spin_unlock_irqrestore(&pool->lock, flags);
 
-	if (bio_list_empty(&bios) && !need_commit_due_to_time(pool))
+	if (bio_list_empty(&bios) &&
+	    !(dm_pool_changed_this_transaction(pool->pmd) && need_commit_due_to_time(pool)))
 		return;
 
 	if (commit(pool)) {