netfilter: nf_ct_dccp/sctp: fix memory leak after netns cleanup

netfilter: nf_ct_dccp/sctp: fix memory leak after netns cleanup

After running the following commands for a while, kmemleak reported that
"1879 new suspected memory leaks" happened:
  # while : ; do
  ip netns add test
  ip netns delete test
  done

  unreferenced object 0xffff88006342fa38 (size 1024):
  comm "ip", pid 15477, jiffies 4295982857 (age 957.836s)
  hex dump (first 32 bytes):
    b8 b0 4d a0 ff ff ff ff c0 34 c3 59 00 88 ff ff  ..M......4.Y....
    04 00 00 00 a4 01 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff8190510a>] kmemleak_alloc+0x4a/0xa0
    [<ffffffff81284130>] __kmalloc_track_caller+0x150/0x300
    [<ffffffff812302d0>] kmemdup+0x20/0x50
    [<ffffffffa04d598a>] dccp_init_net+0x8a/0x160 [nf_conntrack]
    [<ffffffffa04cf9f5>] nf_ct_l4proto_pernet_register_one+0x25/0x90
  ...
  unreferenced object 0xffff88006342da58 (size 1024):
  comm "ip", pid 15477, jiffies 4295982857 (age 957.836s)
  hex dump (first 32 bytes):
    10 b3 4d a0 ff ff ff ff 04 35 c3 59 00 88 ff ff  ..M......5.Y....
    04 00 00 00 a4 01 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<ffffffff8190510a>] kmemleak_alloc+0x4a/0xa0
    [<ffffffff81284130>] __kmalloc_track_caller+0x150/0x300
    [<ffffffff812302d0>] kmemdup+0x20/0x50
    [<ffffffffa04d6a9d>] sctp_init_net+0x5d/0x130 [nf_conntrack]
    [<ffffffffa04cf9f5>] nf_ct_l4proto_pernet_register_one+0x25/0x90
  ...

This is because we forgot to implement the get_net_proto for sctp and
dccp, so we won't invoke the nf_ct_unregister_sysctl to free the
ctl_table when do netns cleanup. Also note, we will fail to register
the sysctl for dccp/sctp either due to the lack of get_net_proto.

Fixes: c51d39010a1b ("netfilter: conntrack: built-in support for DCCP")
Fixes: a85406afeb3e ("netfilter: conntrack: built-in support for SCTP")
Cc: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Acked-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>