netfilter: nft_compat: fix handling of large matchinfo size

currently matchinfo gets stored in the expression, but some xt matches
are very large.

To handle those we either need to switch nft core to kvmalloc and increase
size limit, or allocate the info blob of large matches separately.

This does the latter, this limits the scope of the changes to
nft_compat.

I picked a threshold of 192, this allows most matches to work as before and
handle only few ones via separate alloation (cgroup, u32, sctp, rt).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>