netfilter: xt_CT: Reject the non-null terminated string from user space

The helper and timeout strings are from user-space, we need to make
sure they are null terminated. If not, evil user could make kernel
read the unexpected memory, even print it when fail to find by the
following codes.

pr_info_ratelimited("No such helper \"%s\"\n", helper_name);

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>