memcg: fix build warning and avoid checking for mem != null again and again

Fix build warning, "mem_cgroup_is_obsolete defined but not used" when
CONFIG_DEBUG_VM is not set.  Also avoid checking for !mem again and again.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>