netfilter: xt_socket: Fix broken IPv6 handling

Commit 834184b1 ("netfilter: defrag: only register defrag
functionality if needed") used the outdated XT_SOCKET_HAVE_IPV6 macro
which was removed earlier in commit 8db4c5be ("netfilter: move
socket lookup infrastructure to nf_socket_ipv{4,6}.c"). With that macro
never being defined, the xt_socket match emits an "Unknown family 10"
warning when used with IPv6:

WARNING: CPU: 0 PID: 1377 at net/netfilter/xt_socket.c:160 socket_mt_enable_defrag+0x47/0x50 [xt_socket]
Unknown family 10
Modules linked in: xt_socket nf_socket_ipv4 nf_socket_ipv6 nf_defrag_ipv4 [...]
CPU: 0 PID: 1377 Comm: ip6tables-resto Not tainted 4.10.10 #1
Hardware name: [...]
Call Trace:
? __warn+0xe7/0x100
? socket_mt_enable_defrag+0x47/0x50 [xt_socket]
? socket_mt_enable_defrag+0x47/0x50 [xt_socket]
? warn_slowpath_fmt+0x39/0x40
? socket_mt_enable_defrag+0x47/0x50 [xt_socket]
? socket_mt_v2_check+0x12/0x40 [xt_socket]
? xt_check_match+0x6b/0x1a0 [x_tables]
? xt_find_match+0x93/0xd0 [x_tables]
? xt_request_find_match+0x20/0x80 [x_tables]
? translate_table+0x48e/0x870 [ip6_tables]
? translate_table+0x577/0x870 [ip6_tables]
? walk_component+0x3a/0x200
? kmalloc_order+0x1d/0x50
? do_ip6t_set_ctl+0x181/0x490 [ip6_tables]
? filename_lookup+0xa5/0x120
? nf_setsockopt+0x3a/0x60
? ipv6_setsockopt+0xb0/0xc0
? sock_common_setsockopt+0x23/0x30
? SyS_socketcall+0x41d/0x630
? vfs_read+0xfa/0x120
? do_fast_syscall_32+0x7a/0x110
? entry_SYSENTER_32+0x47/0x71

This patch brings the conditional back in line with how the rest of the
file handles IPv6.

Fixes: 834184b1 ("netfilter: defrag: only register defrag functionality if needed")
Signed-off-by: Peter Tirsek <peter@tirsek.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_socket.c

@@ -152,7 +152,7 @@ static int socket_mt_enable_defrag(struct net *net, int family)
 	switch (family) {
 	case NFPROTO_IPV4:
 		return nf_defrag_ipv4_enable(net);
-#ifdef XT_SOCKET_HAVE_IPV6
+#if IS_ENABLED(CONFIG_IP6_NF_IPTABLES)
 	case NFPROTO_IPV6:
 		return nf_defrag_ipv6_enable(net);
 #endif