netns: Fix crash by making igmp per namespace

This patch makes the multicast socket to be per namespace.

When a network namespace is created, other than the init_net and a
multicast packet is received, the kernel goes to a hang or a kernel panic.

How to reproduce ?

 * create a child network namespace
 * create a pair virtual device veth
    * ip link add type veth
 * move one side to the pair network device to the child namespace
    * ip link set netns <childpid> dev veth1
 * ping -I veth0 224.0.0.1

The bug appears because the function ip_mc_init_dev does not initialize
the different multicast fields as it exits because it is not the init_net.

BUG: soft lockup - CPU#0 stuck for 61s! [avahi-daemon:2695]
Modules linked in:
irq event stamp: 50350
hardirqs last  enabled at (50349): [<c03ee949>] _spin_unlock_irqrestore+0x34/0x39
hardirqs last disabled at (50350): [<c03ec639>] schedule+0x9f/0x5ff
softirqs last  enabled at (45712): [<c0374d4b>] ip_setsockopt+0x8e7/0x909
softirqs last disabled at (45710): [...