memcg: avoid vmpressure oops when memcg disabled

A CONFIG_MEMCG=y kernel booted with "cgroup_disable=memory" crashes on a
NULL memcg (but non-NULL root_mem_cgroup) when vmpressure kicks in.
Here's the patch I use to avoid that, but you might prefer a test on
mem_cgroup_disabled() somewhere.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>