netfilter: nf_conntrack: register pernet subsystem before register L4 proto

In (c296bb4d netfilter: nf_conntrack: refactor l4proto support for netns)
the l4proto gre/dccp/udplite/sctp registration happened before the pernet
subsystem, which is wrong.

Register pernet subsystem before register L4proto since after register
L4proto, init_conntrack may try to access the resources which allocated
in register_pernet_subsys.

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>