[NETFILTER]: nf_conntrack: remove ugly hack in l4proto registration

Remove ugly special-casing of nf_conntrack_l4proto_generic, all it
wants is its sysctl tables registered, so do that explicitly in an
init function and move the remaining protocol initialization and
cleanup code to nf_conntrack_proto.c as well.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>