[NETNS]: Pass correct namespace in ip_rt_get_source.

ip_rt_get_source is the infamous place for which dst_ifdown kludges
have been implemented. This means that rt->u.dst.dev can be safely
dereferrenced obtain nd_net.
Signed-off-by: NDenis V. Lunev <den@openvz.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1559,7 +1559,7 @@ void ip_rt_get_source(u8 *addr, struct rtable *rt)
 
 	if (rt->fl.iif == 0)
 		src = rt->rt_src;
-	else if (fib_lookup(&init_net, &rt->fl, &res) == 0) {
+	else if (fib_lookup(rt->u.dst.dev->nd_net, &rt->fl, &res) == 0) {
 		src = FIB_RES_PREFSRC(res);
 		fib_res_put(&res);
 	} else