udp6: allow SO_MARK ctrl msg to affect routing

Commit c6af0c22 ("ip: support SO_MARK cmsg")
added propagation of SO_MARK from cmsg to skb->mark.
For IPv4 and raw sockets the mark also affects route
lookup, but in case of IPv6 the flow info is
initialized before cmsg is parsed.

Fixes: c6af0c22 ("ip: support SO_MARK cmsg")
Reported-and-tested-by: NXintong Hu <huxintong@fb.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>
Reviewed-by: NDavid Ahern <dsahern@kernel.org>
Reviewed-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -1435,7 +1435,6 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	if (!fl6.flowi6_oif)
 		fl6.flowi6_oif = np->sticky_pktinfo.ipi6_ifindex;
 
-	fl6.flowi6_mark = ipc6.sockc.mark;
 	fl6.flowi6_uid = sk->sk_uid;
 
 	if (msg->msg_controllen) {
@@ -1471,6 +1470,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	ipc6.opt = opt;
 
 	fl6.flowi6_proto = sk->sk_protocol;
+	fl6.flowi6_mark = ipc6.sockc.mark;
 	fl6.daddr = *daddr;
 	if (ipv6_addr_any(&fl6.saddr) && !ipv6_addr_any(&np->saddr))
 		fl6.saddr = np->saddr;