netns: igmp: make /proc/net/{igmp,mcfilter} per netns

This patch makes the followinf proc entries per-netns:
/proc/net/igmp
/proc/net/mcfilter

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>