ipv6: set msg_control_is_user in do_ipv6_getsockopt

While do_ipv6_getsockopt does not call the high-level recvmsg helper,
the msghdr eventually ends up being passed to put_cmsg anyway, and thus
needs msg_control_is_user set to the proper value.

Fixes: 1f466e1 ("net: cleanly handle kernel vs user buffers for ->msg_control")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -1075,6 +1075,7 @@ static int do_ipv6_getsockopt(struct sock *sk, int level, int optname,
 		msg.msg_control = optval;
 		msg.msg_controllen = len;
 		msg.msg_flags = flags;
+		msg.msg_control_is_user = true;
 
 		lock_sock(sk);
 		skb = np->pktoptions;