netfilter: nf_ct_h323: cap packet size at 64k

With BIG TCP, packets generated by tcp stack may exceed 64kb.
Cap datalen at 64kb.  The internal message format uses 16bit fields,
so no embedded message can exceed 64k size.

Multiple h323 messages in a single superpacket may now result
in a message to get treated as incomplete/truncated, but thats
better than scribbling past h323_buffer.

Another alternative suitable for net tree would be a switch to
skb_linearize().

Fixes: 7c4e983c ("net: allow gso_max_size to exceed 65536")
Fixes: 0fe79f28 ("net: allow gro_max_size to exceed 65536")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>