tun: Set net ns before opening master

Opening tun device is tricky: the net device
is allocated from current net namespace, thus
we should set it first, otherwise all tuns
which live in nested net namespace get moved
to the toplevel one.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>

criu/tun.c

@@ -369,13 +369,14 @@ static int tunfile_open(struct file_desc *d, int *new_fd)
 	struct tun_link *tl;
 
 	ti = container_of(d, struct tunfile_info, d);
-	fd = open_reg_by_id(ti->tfe->id);
-	if (fd < 0)
-		return -1;
 
 	ns_id = ti->tfe->ns_id;
 	if (set_netns(ns_id))
-		goto err;
+		return -1;
+
+	fd = open_reg_by_id(ti->tfe->id);
+	if (fd < 0)
+		return -1;
 
 	if (!ti->tfe->netdev)
 		/* just-opened tun file */