mm, memcg: fix build error around the usage of kmem_caches

mm, memcg: fix build error around the usage of kmem_caches

When I manually set default n to MEMCG_KMEM in init/Kconfig, bellow error
occurs,

  mm/slab_common.c: In function 'memcg_slab_start':
  mm/slab_common.c:1530:30: error: 'struct mem_cgroup' has no member named
  'kmem_caches'
    return seq_list_start(&memcg->kmem_caches, *pos);
                                ^
  mm/slab_common.c: In function 'memcg_slab_next':
  mm/slab_common.c:1537:32: error: 'struct mem_cgroup' has no member named
  'kmem_caches'
    return seq_list_next(p, &memcg->kmem_caches, pos);
                                  ^
  mm/slab_common.c: In function 'memcg_slab_show':
  mm/slab_common.c:1551:16: error: 'struct mem_cgroup' has no member named
  'kmem_caches'
    if (p == memcg->kmem_caches.next)
                  ^
    CC      arch/x86/xen/smp.o
  mm/slab_common.c: In function 'memcg_slab_start':
  mm/slab_common.c:1531:1: warning: control reaches end of non-void function
  [-Wreturn-type]
   }
   ^
  mm/slab_common.c: In function 'memcg_slab_next':
  mm/slab_common.c:1538:1: warning: control reaches end of non-void function
  [-Wreturn-type]
   }
   ^

That's because kmem_caches is defined only when CONFIG_MEMCG_KMEM is set,
while memcg_slab_start() will use it no matter CONFIG_MEMCG_KMEM is defined
or not.

By the way, the reason I mannuly undefined CONFIG_MEMCG_KMEM is to verify
whether my some other code change is still stable when CONFIG_MEMCG_KMEM is
not set. Unfortunately, the existing code has been already unstable since
v4.11.

Fixes: bc2791f857e1 ("slab: link memcg kmem_caches on their associated memory cgroup")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Link: http://lkml.kernel.org/r/1580970260-2045-1-git-send-email-laoar.shao@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>