netfilter: cttimeout: fix buffer overflow

Chen Gang reports:
the length of nla_data(cda[CTA_TIMEOUT_NAME]) is not limited in server side.

And indeed, its used to strcpy to a fixed-sized buffer.

Fortunately, nfnetlink users need CAP_NET_ADMIN.

Reported-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>