ipv6: fix inet6_csk_update_pmtu() return value

In case of error, inet6_csk_update_pmtu() should consistently
return NULL.

Bug added in commit 35ad9b9c
(ipv6: Add helper inet6_csk_update_pmtu().)
Reported-by: Lluís Batlle i Rossell <viric@viric.name>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/inet6_connection_sock.c

@@ -252,6 +252,7 @@ struct dst_entry *inet6_csk_update_pmtu(struct sock *sk, u32 mtu)
 		return NULL;
 	dst->ops->update_pmtu(dst, sk, NULL, mtu);
 
-	return inet6_csk_route_socket(sk, &fl6);
+	dst = inet6_csk_route_socket(sk, &fl6);
+	return IS_ERR(dst) ? NULL : dst;
 }
 EXPORT_SYMBOL_GPL(inet6_csk_update_pmtu);