memcontrol: add vm_match_cgroup()

mm_cgroup() is exclusively used to test whether an mm's mem_cgroup pointer
is pointing to a specific cgroup.  Instead of returning the pointer, we can
just do the test itself in a new macro:

	vm_match_cgroup(mm, cgroup)

returns non-zero if the mm's mem_cgroup points to cgroup.  Otherwise it
returns zero.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -399,7 +399,7 @@ int task_in_mem_cgroup(struct task_struct *task, const struct mem_cgroup *mem)
 	int ret;
 
 	task_lock(task);
-	ret = task->mm && mm_cgroup(task->mm) == mem;
+	ret = task->mm && vm_match_cgroup(task->mm, mem);
 	task_unlock(task);
 	return ret;
 }