-rt: lockdep warning mc_list_lock

[Daniel Walker <dwalker@mvista.com>]

Got this warning yesterday.

=============================================
[ INFO: possible recursive locking detected ]
---------------------------------------------
softirq-timer/1/17 is trying to acquire lock:
 (&in_dev->mc_list_lock){--..}, at: [<c03e919e>] ip_check_mc+0x1e/0xb0

but task is already holding lock:
 (&in_dev->mc_list_lock){--..}, at: [<c03ec4a7>] igmp_ifc_timer_expire+0x17/0x2a0

other info that might help us debug this:
2 locks held by softirq-timer/1/17:
 #0:  (&in_dev->mc_list_lock){--..}, at: [<c03ec4a7>] igmp_ifc_timer_expire+0x17/0x2a0
 #1:  (&im->lock){--..}, at: [<c03ec5ff>] igmp_ifc_timer_expire+0x16f/0x2a0

stack backtrace:
 [<c0105dcb>] show_trace+0x1b/0x20
 [<c0105df4>] dump_stack+0x24/0x30
 [<c013e4fb>] __lock_acquire+0xb1b/0xe40
 [<c013eb82>] lock_acquire+0x62/0x80
 [<c041b736>] rt_read_lock+0x36/0x80
 [<c03e919e>] ip_check_mc+0x1e/0xb0
 [<c03bc3f5>] __ip_route_output_key+0x6b5/0x880
 [<c03bc5de>] ip_route_output_flow+0x1e/0x80
 [<c03bc664>] ip_route_output_key+0x24/0x30
 [<c03eaa91>] igmpv3_newpack+0x71/0x270
 [<c03ead2d>] add_grhead+0x9d/0x100
 [<c03eb17d>] add_grec+0x30d/0x450
 [<c03ec5da>] igmp_ifc_timer_expire+0x14a/0x2a0
 [<c012ae1a>] run_timer_softirq+0x10a/0xd40
 [<c0126235>] ksoftirqd+0x125/0x200
 [<c013561d>] kthread+0xfd/0x110
 [<c0100f25>] kernel_thread_helper+0x5/0x10
---------------------------
| preempt count: 00000000 ]
| 0-level deep critical section nesting:
----------------------------------------


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