netfilter: nf_conntrack: move initialization out of pernet operations

nf_conntrack initialization and cleanup codes happens in pernet
operations function. This task should be done in module_init/exit.
We can't use init_net to identify if it's the right time to initialize
or cleanup since we cannot make assumption on the order netns are
created/destroyed.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>