btrfs: avoid starting new transaction when flushing delayed items and refs

When flushing space we join a transaction to flush delayed items and
delayed references, in order to try to release space. However using
btrfs_join_transaction() not only joins an existing transaction as well
as it starts a new transaction if there is none open. If there is no
transaction open, we don't have neither delayed items nor delayed
references, so creating a new transaction is a waste of time, IO and
creates an unnecessary rotation of the backup roots without gaining any
benefits (including releasing space).

So use btrfs_join_transaction_nostart() when attempting to flush delayed
items and references.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/space-info.c

@@ -725,9 +725,11 @@ static void flush_space(struct btrfs_fs_info *fs_info,
 		else
 			nr = -1;
 
-		trans = btrfs_join_transaction(root);
+		trans = btrfs_join_transaction_nostart(root);
 		if (IS_ERR(trans)) {
 			ret = PTR_ERR(trans);
+			if (ret == -ENOENT)
+				ret = 0;
 			break;
 		}
 		ret = btrfs_run_delayed_items_nr(trans, nr);
@@ -743,9 +745,11 @@ static void flush_space(struct btrfs_fs_info *fs_info,
 		break;
 	case FLUSH_DELAYED_REFS_NR:
 	case FLUSH_DELAYED_REFS:
-		trans = btrfs_join_transaction(root);
+		trans = btrfs_join_transaction_nostart(root);
 		if (IS_ERR(trans)) {
 			ret = PTR_ERR(trans);
+			if (ret == -ENOENT)
+				ret = 0;
 			break;
 		}
 		if (state == FLUSH_DELAYED_REFS_NR)