[NETNS][ICMP]: Use per-net sysctls in ipv4/icmp.c.

This mostly re-uses the net, used in icmp netnsization patches from Denis.

After this ICMP sysctls are completely virtualized.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>