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): [<c03ee682>] _spin_lock_bh+0x8/0x27

Pid: 2695, comm: avahi-daemon Not tainted (2.6.27-rc2-00029-g0872073 #3)
EIP: 0060:[<c03ee47c>] EFLAGS: 00000297 CPU: 0
EIP is at __read_lock_failed+0x8/0x10
EAX: c4f38810 EBX: c4f38810 ECX: 00000000 EDX: c04cc22e
ESI: fb0000e0 EDI: 00000011 EBP: 0f02000a ESP: c4e3faa0
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
CR0: 8005003b CR2: 44618a40 CR3: 04e37000 CR4: 000006d0
DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
DR6: ffff0ff0 DR7: 00000400
 [<c02311f8>] ? _raw_read_lock+0x23/0x25
 [<c0390666>] ? ip_check_mc+0x1c/0x83
 [<c036d478>] ? ip_route_input+0x229/0xe92
 [<c022e2e4>] ? trace_hardirqs_on_thunk+0xc/0x10
 [<c0104c9c>] ? do_IRQ+0x69/0x7d
 [<c0102e64>] ? restore_nocheck_notrace+0x0/0xe
 [<c036fdba>] ? ip_rcv+0x227/0x505
 [<c0358764>] ? netif_receive_skb+0xfe/0x2b3
 [<c03588d2>] ? netif_receive_skb+0x26c/0x2b3
 [<c035af31>] ? process_backlog+0x73/0xbd
 [<c035a8cd>] ? net_rx_action+0xc1/0x1ae
 [<c01218a8>] ? __do_softirq+0x7b/0xef
 [<c0121953>] ? do_softirq+0x37/0x4d
 [<c035b50d>] ? dev_queue_xmit+0x3d4/0x40b
 [<c0122037>] ? local_bh_enable+0x96/0xab
 [<c035b50d>] ? dev_queue_xmit+0x3d4/0x40b
 [<c012181e>] ? _local_bh_enable+0x79/0x88
 [<c035fcb8>] ? neigh_resolve_output+0x20f/0x239
 [<c0373118>] ? ip_finish_output+0x1df/0x209
 [<c0373364>] ? ip_dev_loopback_xmit+0x62/0x66
 [<c0371db5>] ? ip_local_out+0x15/0x17
 [<c0372013>] ? ip_push_pending_frames+0x25c/0x2bb
 [<c03891b8>] ? udp_push_pending_frames+0x2bb/0x30e
 [<c038a189>] ? udp_sendmsg+0x413/0x51d
 [<c038a1a9>] ? udp_sendmsg+0x433/0x51d
 [<c038f927>] ? inet_sendmsg+0x35/0x3f
 [<c034f092>] ? sock_sendmsg+0xb8/0xd1
 [<c012d554>] ? autoremove_wake_function+0x0/0x2b
 [<c022e6de>] ? copy_from_user+0x32/0x5e
 [<c022e6de>] ? copy_from_user+0x32/0x5e
 [<c034f238>] ? sys_sendmsg+0x18d/0x1f0
 [<c0175e90>] ? pipe_write+0x3cb/0x3d7
 [<c0170347>] ? do_sync_write+0xbe/0x105
 [<c012d554>] ? autoremove_wake_function+0x0/0x2b
 [<c03503b2>] ? sys_socketcall+0x176/0x1b0
 [<c01085ea>] ? syscall_trace_enter+0x6c/0x7b
 [<c0102e1a>] ? syscall_call+0x7/0xb

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>