memcg: check that kmem_cache has memcg_params before accessing it

If the system had a few memory groups and all of them were destroyed,
memcg_limited_groups_array_size has non-zero value, but all new caches
are created without memcg_params, because memcg_kmem_enabled() returns
false.

We try to enumirate child caches in a few places and all of them are
potentially dangerous.

For example my kernel is compiled with CONFIG_SLAB and it crashed when I
tryed to mount a NFS share after a few experiments with kmemcg.

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
  IP: [<ffffffff8118166a>] do_tune_cpucache+0x8a/0xd0
  PGD b942a067 PUD b999f067 PMD 0
  Oops: 0000 [#1] SMP
  Modules linked in: fscache(+) ip6table_filter ip6_tables iptable_filter ip_tables i2c_piix4 pcspkr virtio_net virtio_balloon i2c_core floppy
  CPU: 0 PID: 357 Comm: modprobe Not tainted 3.11.0-rc7+ #59
  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
  task: ffff8800b9f98240 ti: ffff8800ba32e000 task.ti: ffff8800ba32e000
  RIP: 0010:[<ffffffff8118166a>]  [<ffffffff8118166a>] do_tune_cpucache+0x8a/0xd0
  RSP: 0018:ffff8800ba32fb70  EFLAGS: 00010246
  RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000006
  RDX: 0000000000000000 RSI: ffff8800b9f98910 RDI: 0000000000000246
  RBP: ffff8800ba32fba0 R08: 0000000000000002 R09: 0000000000000004
  R10: 0000000000000001 R11: 0000000000000001 R12: 0000000000000010
  R13: 0000000000000008 R14: 00000000000000d0 R15: ffff8800375d0200
  FS:  00007f55f1378740(0000) GS:ffff8800bfa00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
  CR2: 00007f24feba57a0 CR3: 0000000037b51000 CR4: 00000000000006f0
  Call Trace:
    enable_cpucache+0x49/0x100
    setup_cpu_cache+0x215/0x280
    __kmem_cache_create+0x2fa/0x450
    kmem_cache_create_memcg+0x214/0x350
    kmem_cache_create+0x2b/0x30
    fscache_init+0x19b/0x230 [fscache]
    do_one_initcall+0xfa/0x1b0
    load_module+0x1c41/0x26d0
    SyS_finit_module+0x86/0xb0
    system_call_fastpath+0x16/0x1b

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Glauber Costa <glommer@openvz.org>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>