[NETFILTER] ctnetlink: ICMP ID is not mandatory

The ID is only required by ICMP type 8 (echo), so it's not
mandatory for all sort of ICMP connections. This patch makes
mandatory only the type and the code for ICMP netlink messages.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>