zdtm: Make /dev/net/tun optional device in test ns

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/ns.c

@@ -366,13 +366,14 @@ static int construct_root()
 	chmod("dev", 0777);
 	mknod("dev/null", 0777 | S_IFCHR, makedev(1, 3));
 	chmod("dev/null", 0777);
-	mkdir("dev/net", 0777);
-	if (stat("/dev/net/tun", &st)) {
+	if (stat("/dev/net/tun", &st))
 		fprintf(stderr, "Unable to stat /dev/net/tun: %m");
-		return -1;
-	}
+	else {
+		mkdir("dev/net", 0777);
 		mknod("dev/net/tun", 0777 | S_IFCHR, st.st_rdev);
 		chmod("dev/net/tun", 0777);
+	}
+
 	if (stat("/dev/rtc", &st)) {
 		fprintf(stderr, "Unable to stat /dev/rtc: %m");
 		return -1;