Btrfs: update nodatacow code v2

This patch simplifies the nodatacow checker. If all references
were created after the latest snapshot, then we can avoid COW
safely. This patch also updates run_delalloc_nocow to do more
fine-grained checking.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>

fs/btrfs/transaction.c

@@ -763,6 +763,8 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
 	if (ret)
 		goto fail;
 
+	btrfs_record_root_in_trans(root);
+	btrfs_set_root_last_snapshot(&root->root_item, trans->transid);
 	memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
 
 	key.objectid = objectid;