cgroup: Fix uninitialized variable warning

Commit 1f7dd3e ("cgroup: fix handling of multi-destination migration
from subtree_control enabling") introduced the following compiler warning:

mm/memcontrol.c: In function ‘mem_cgroup_can_attach’:
mm/memcontrol.c:4790:9: warning: ‘memcg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
   mc.to = memcg;
         ^

Fix this by initializing 'memcg' to NULL.

This was found using gcc (GCC) 4.9.2 20150212 (Red Hat 4.9.2-6).

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

mm/memcontrol.c

@@ -4782,7 +4782,7 @@ static void mem_cgroup_clear_mc(void)
 static int mem_cgroup_can_attach(struct cgroup_taskset *tset)
 {
 	struct cgroup_subsys_state *css;
-	struct mem_cgroup *memcg;
+	struct mem_cgroup *memcg = NULL; /* unneeded init to make gcc happy */
 	struct mem_cgroup *from;
 	struct task_struct *leader, *p;
 	struct mm_struct *mm;