netfilter: conntrack: register sysctl table for gre

This patch adds two sysctl knobs for GRE:

	net.netfilter.nf_conntrack_gre_timeout = 30
	net.netfilter.nf_conntrack_gre_timeout_stream = 180

Update the Documentation as well.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>