netfilter: nf_ct_ftp: prefer skb_linearize

This uses a pseudo-linearization scheme with a 64k global buffer,
but BIG TCP arrival means IPv6 TCP stack can generate skbs
that exceed this size.

Use skb_linearize.  It should be possible to rewrite this to properly
deal with segmented skbs (i.e., only do small chunk-wise accesses),
but this is going to be a lot more intrusive than this because every
helper function needs to get the sk_buff instead of a pointer to a raw
data buffer.

In practice, provided we're really looking at FTP control channel packets,
there should never be a case where we deal with huge packets.

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>