netfilter: nf_ct_gre: use new namespace support

This patch modifies the GRE protocol tracker, which partially
supported namespace before this patch, to use the new namespace
infrastructure for nf_conntrack.

Acked-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>