memcontrol: move mm_cgroup to header file

Inline functions must preceed their use, so mm_cgroup() should be defined
in linux/memcontrol.h.

include/linux/memcontrol.h:48: warning: 'mm_cgroup' declared inline after
	being called
include/linux/memcontrol.h:48: warning: previous declaration of
	'mm_cgroup' was here

[akpm@linux-foundation.org: build fix]
[akpm@linux-foundation.org: nuther build fix]
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>