memcg: NULL pointer dereference at rmdir on some NUMA systems

N_POSSIBLE doesn't means there is memory...and force_empty can
visit invalid node which have no pgdat.

To visit all valid nodes, N_HIGH_MEMORY should be used.

Reported-by: Li Zefan <[e-mail]>
Signed-off-by: KAMEZAWA Hiroyuki <[e-mail]>
Tested-by: Li Zefan <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Cc: Daisuke Nishimura <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 		/* This is for making all *used* pages to be on LRU. */
 		lru_add_drain_all();
 		ret = 0;
-		for_each_node_state(node, N_POSSIBLE) {
+		for_each_node_state(node, N_HIGH_MEMORY) {
 			for (zid = 0; !ret && zid < MAX_NR_ZONES; zid++) {
 				enum lru_list l;
 				for_each_lru(l) {