netfilter: xt_CT: refactorize xt_ct_tg_check

This patch adds xt_ct_set_helper and xt_ct_set_timeout to reduce
the size of xt_ct_tg_check.

This aims to improve code mantainability by splitting xt_ct_tg_check
in smaller chunks.

Suggested by Eric Dumazet.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>