netfilter: connlimit: merge root4 and root6.

The root4 variable is used only when connlimit extension module has been
stored by the iptables command. and the roo6 variable is used only when
connlimit extension module has been stored by the ip6tables command.
So the root4 and roo6 variable does not be used at the same time.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>