memcg, slab: do not destroy children caches if parent has aliases

Currently we destroy children caches at the very beginning of
kmem_cache_destroy().  This is wrong, because the root cache will not
necessarily be destroyed in the end - if it has aliases (refcount > 0),
kmem_cache_destroy() will simply decrement its refcount and return.  In
this case, at best we will get a bunch of warnings in dmesg, like this
one:

  kmem_cache_destroy kmalloc-32:0: Slab cache still has objects
  CPU: 1 PID: 7139 Comm: modprobe Tainted: G    B   W    3.13.0+ #117
  Call Trace:
    dump_stack+0x49/0x5b
    kmem_cache_destroy+0xdf/0xf0
    kmem_cache_destroy_memcg_children+0x97/0xc0
    kmem_cache_destroy+0xf/0xf0
    xfs_mru_cache_uninit+0x21/0x30 [xfs]
    exit_xfs_fs+0x2e/0xc44 [xfs]
    SyS_delete_module+0x198/0x1f0
    system_call_fastpath+0x16/0x1b

At worst - if kmem_cache_destroy() will race with an allocation from a
memcg cache - the kernel will panic.

This patch fixes this by moving children caches destruction after the
check if the cache has aliases.  Plus, it forbids destroying a root
cache if it still has children caches, because each children cache keeps
a reference to its parent.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Glauber Costa <glommer@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>