IPv6: fix Mobile IPv6 regression

Commit f4f914b5 (net: ipv6 bind to device issue) caused
a regression with Mobile IPv6 when it changed the meaning
of fl->oif to become a strict requirement of the route
lookup.  Instead, only force strict mode when
sk->sk_bound_dev_if is set on the calling socket, getting
the intended behavior and fixing the regression.
Tested-by: NArnaud Ebalard <arno@natisbad.org>
Signed-off-by: NBrian Haley <brian.haley@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -814,7 +814,7 @@ struct dst_entry * ip6_route_output(struct net *net, struct sock *sk,
 {
 	int flags = 0;
 
-	if (fl->oif || rt6_need_strict(&fl->fl6_dst))
+	if ((sk && sk->sk_bound_dev_if) || rt6_need_strict(&fl->fl6_dst))
 		flags |= RT6_LOOKUP_F_IFACE;
 
 	if (!ipv6_addr_any(&fl->fl6_src))