[NETFILTER]: ctnetlink: clear helper area and handle unchanged helper

This patch
- Clears private area for helper even if no helper is assigned to
  conntrack. It might be used by old helper.
- Unchanges if the same helper as the used one is specified.
- Does not find helper if no helper is specified. And it does not
  require private area for helper in that case.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>