tun: don't leak a file descriptor

CID 84654 (#1 of 1): Resource leak (RESOURCE_LEAK)
6. leaked_handle: Handle variable fd going out of scope leaks the handle.
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/tun.c

@@ -371,7 +371,7 @@ static int tunfile_open(struct file_desc *d, int *new_fd)
 
 	ns_id = ti->tfe->ns_id;
 	if (set_netns(ns_id))
-		return -1;
+		goto err;
 
 	if (!ti->tfe->netdev)
 		/* just-opened tun file */