SLAB: Fix lockdep annotations for CPU hotplug

As reported by Paul McKenney:

  I am seeing some lockdep complaints in rcutorture runs that include
  frequent CPU-hotplug operations.  The tests are otherwise successful.
  My first thought was to send a patch that gave each array_cache
  structure's ->lock field its own struct lock_class_key, but you already
  have a init_lock_keys() that seems to be intended to deal with this.

  ------------------------------------------------------------------------

  =============================================
  [ INFO: possible recursive locking detected ]
  2.6.32-rc4-autokern1 #1
  ---------------------------------------------
  syslogd/2908 is trying to acquire lock:
   (&nc->lock){..-...}, at: [<c0000000001407f4>] .kmem_cache_free+0x118/0x2d4

  but task is already holding lock:
   (&nc->lock){..-...}, at: [<c0000000001411bc>] .kfree+0x1f0/0x324

  other info that might help us debug this:
  3 locks held by syslogd/2908:
   #0:  (&u->readlock){+.+.+.}, at: [<c0000000004556f8>] .unix_dgram_recvmsg+0x70/0x338
   #1:  (&nc->lock){..-...}, at: [<c0000000001411bc>] .kfree+0x1f0/0x324
   #2:  (&parent->list_lock){-.-...}, at: [<c000000000140f64>] .__drain_alien_cache+0x50/0xb8

  stack backtrace:
  Call Trace:
  [c0000000e8ccafc0] [c0000000000101e4] .show_stack+0x70/0x184 (unreliable)
  [c0000000e8ccb070] [c0000000000afebc] .validate_chain+0x6ec/0xf58
  [c0000000e8ccb180] [c0000000000b0ff0] .__lock_acquire+0x8c8/0x974
  [c0000000e8ccb280] [c0000000000b2290] .lock_acquire+0x140/0x18c
  [c0000000e8ccb350] [c000000000468df0] ._spin_lock+0x48/0x70
  [c0000000e8ccb3e0] [c0000000001407f4] .kmem_cache_free+0x118/0x2d4
  [c0000000e8ccb4a0] [c000000000140b90] .free_block+0x130/0x1a8
  [c0000000e8ccb540] [c000000000140f94] .__drain_alien_cache+0x80/0xb8
  [c0000000e8ccb5e0] [c0000000001411e0] .kfree+0x214/0x324
  [c0000000e8ccb6a0] [c0000000003ca860] .skb_release_data+0xe8/0x104
  [c0000000e8ccb730] [c0000000003ca2ec] .__kfree_skb+0x20/0xd4
  [c0000000e8ccb7b0] [c0000000003cf2c8] .skb_free_datagram+0x1c/0x5c
  [c0000000e8ccb830] [c00000000045597c] .unix_dgram_recvmsg+0x2f4/0x338
  [c0000000e8ccb920] [c0000000003c0f14] .sock_recvmsg+0xf4/0x13c
  [c0000000e8ccbb30] [c0000000003c28ec] .SyS_recvfrom+0xb4/0x130
  [c0000000e8ccbcb0] [c0000000003bfb78] .sys_recv+0x18/0x2c
  [c0000000e8ccbd20] [c0000000003ed388] .compat_sys_recv+0x14/0x28
  [c0000000e8ccbd90] [c0000000003ee1bc] .compat_sys_socketcall+0x178/0x220
  [c0000000e8ccbe30] [c0000000000085d4] syscall_exit+0x0/0x40

This patch fixes the issue by setting up lockdep annotations during CPU
hotplug.

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>