Btrfs: flush data on snapshot creation

Flush any delalloc extents when we create a snapshot, so that recently
written file data is always included in the snapshot.

A later commit will add the ability to snapshot without the flush, but
most people expect flushing.

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/transaction.c

@@ -997,13 +997,10 @@ int btrfs_commit_transaction(struct btrfs_trans_handle *trans,
 
 		mutex_unlock(&root->fs_info->trans_mutex);
 
-		if (flush_on_commit) {
+		if (flush_on_commit || snap_pending) {
 			btrfs_start_delalloc_inodes(root, 1);
 			ret = btrfs_wait_ordered_extents(root, 0, 1);
 			BUG_ON(ret);
-		} else if (snap_pending) {
-			ret = btrfs_wait_ordered_extents(root, 0, 1);
-			BUG_ON(ret);
 		}
 
 		/*