net: ipv6: Implement /proc/net/icmp6.

The format is based on /proc/net/icmp and /proc/net/{udp,raw}6.

Compiles and displays reasonable results with CONFIG_IPV6={n,m,y}
Couldn't figure out how to test without CONFIG_PROC_FS enabled.

Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>