[NETNS][ICMP]: Make ctl tables for ICMP sysctls per-net.

Add some flesh to ipv4_sysctl_init_net and ipv4_sysctl_exit_net,
i.e. copy the table, alter .data pointers and register it per-net.

Other ipv4_table's sysctls are now global, but this is going to
change once sysctl permissions patches migrate from -mm tree to 
mainline in 2.6.26 merge window :)

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