netfilter: nf_conntrack_h323: lost .data_len definition for Q.931/ipv6

BugLink: https://bugs.launchpad.net/bugs/1887011

Could you please push this patch into stable@?
it fixes memory corruption in kernels  v3.5 .. v4.10

Lost .data_len definition leads to write beyond end of
struct nf_ct_h323_master. Usually it corrupts following
struct nf_conn_nat, however if nat is not loaded it corrupts
following slab object.

In mainline this problem went away in v4.11,
after commit 9f0f3ebe ("netfilter: helpers: remove data_len usage
for inkernel helpers") however many stable kernels are still affected.

Fixes: 1afc5679 ("netfilter: nf_ct_helper: implement variable length helper private data") # v3.5
cc: stable@vger.kernel.org
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/netfilter/nf_conntrack_h323_main.c

@@ -1225,6 +1225,7 @@ static struct nf_conntrack_helper nf_conntrack_helper_q931[] __read_mostly = {
 	{
 		.name			= "Q.931",
 		.me			= THIS_MODULE,
+		.data_len		= sizeof(struct nf_ct_h323_master),
 		.tuple.src.l3num	= AF_INET6,
 		.tuple.src.u.tcp.port	= cpu_to_be16(Q931_PORT),
 		.tuple.dst.protonum	= IPPROTO_TCP,