memcg: initialize kmem-cache destroying work earlier

Fix a warning from lockdep caused by calling cancel_work_sync() for
uninitialized struct work.  This path has been triggered by destructon
kmem-cache hierarchy via destroying its root kmem-cache.

  cache ffff88003c072d80
  obj ffff88003b410000 cache ffff88003c072d80
  obj ffff88003b924000 cache ffff88003c20bd40
  INFO: trying to register non-static key.
  the code is fine but needs lockdep annotation.
  turning off the locking correctness validator.
  Pid: 2825, comm: insmod Tainted: G           O 3.9.0-rc1-next-20130307+ #611
  Call Trace:
    __lock_acquire+0x16a2/0x1cb0
    lock_acquire+0x8a/0x120
    flush_work+0x38/0x2a0
    __cancel_work_timer+0x89/0xf0
    cancel_work_sync+0xb/0x10
    kmem_cache_destroy_memcg_children+0x81/0xb0
    kmem_cache_destroy+0xf/0xe0
    init_module+0xcb/0x1000 [kmem_test]
    do_one_initcall+0x11a/0x170
    load_module+0x19b0/0x2320
    SyS_init_module+0xc6/0xf0
    system_call_fastpath+0x16/0x1b

Example module to de...