net: core: Fix build with CONFIG_IPV6=m

Stephen Rothwell reports the following link failure with IPv6 as module:

  x86_64-linux-gnu-ld: net/core/filter.o: in function `sk_lookup':
  (.text+0x19219): undefined reference to `__udp6_lib_lookup'

Fix the build by only enabling the IPv6 socket lookup if IPv6 support is
compiled into the kernel.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -4838,7 +4838,7 @@ struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
 			sk = __udp4_lib_lookup(net, src4, tuple->ipv4.sport,
 					       dst4, tuple->ipv4.dport,
 					       dif, sdif, &udp_table, skb);
-#if IS_ENABLED(CONFIG_IPV6)
+#if IS_REACHABLE(CONFIG_IPV6)
 	} else {
 		struct in6_addr *src6 = (struct in6_addr *)&tuple->ipv6.saddr;
 		struct in6_addr *dst6 = (struct in6_addr *)&tuple->ipv6.daddr;