tproxy: Add missing CAP_NET_ADMIN check to ipv6 side

IP_TRANSPARENT requires root (more precisely CAP_NET_ADMIN privielges)
for IPV6.

However as I see right now this check was missed from the IPv6
implementation.
Acked-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -343,6 +343,10 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		break;
 
 	case IPV6_TRANSPARENT:
+		if (!capable(CAP_NET_ADMIN)) {
+			retv = -EPERM;
+			break;
+		}
 		if (optlen < sizeof(int))
 			goto e_inval;
 		/* we don't have a separate transparent bit for IPV6 we use the one in the IPv4 socket */