netfilter: nf_conntrack: use l4proto->users as refcount for per-net data

Currently, nf_proto_net's l4proto->users meaning is quite confusing
since it depends on the compilation tweaks.

To resolve this, we cleanup this code to regard it as the refcount
for l4proto's per-net data, since there may be two l4protos use the
same per-net data.

Thus, we increment pn->users when nf_conntrack_l4proto_register
successfully, and decrement it for nf_conntrack_l4_unregister case.

The users refcnt is not required form layer 3 protocol trackers.

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