netfilter: nf_ct_sane: remove pseudo skb linearization

For historical reason this code performs pseudo linearization of skbs
via skb_header_pointer and a global 64k buffer.

With arrival of BIG TCP, packets generated by TCP stack can exceed 64kb.

Rewrite this to only extract the needed header data.  This also allows
to get rid of the locking.

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>