netfilter: nf_ct_sctp: merge sctpv[4,6]_net_init into sctp_net_init

Merge sctpv4_net_init and sctpv6_net_init into sctp_net_init to
remove redundant code now that we have the u_int16_t proto
parameter.

And use nf_proto_net.users to identify if it's the first time
we use the nf_proto_net, in that case, we initialize i

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