ipv6: fix IPV6_PKTINFO with v4 mapped

Use IS_ENABLED(CONFIG_IPV6), to enable this code if IPv6 is
a module.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: c8e6ad0829 ("ipv6: honor IPV6_PKTINFO with v4 mapped addresses on sendmsg")
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -195,7 +195,7 @@ int ip_cmsg_send(struct net *net, struct msghdr *msg, struct ipcm_cookie *ipc,
 	for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg, cmsg)) {
 		if (!CMSG_OK(msg, cmsg))
 			return -EINVAL;
-#if defined(CONFIG_IPV6)
+#if IS_ENABLED(CONFIG_IPV6)
 		if (allow_ipv6 &&
 		    cmsg->cmsg_level == SOL_IPV6 &&
 		    cmsg->cmsg_type == IPV6_PKTINFO) {