memcg: NULL pointer dereference at rmdir on some NUMA systems

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