mm/mempolicy: return NULL if node is NUMA_NO_NODE in get_task_policy

If node == NUMA_NO_NODE, pol is NULL, we should return NULL instead of
do "if (!pol->mode)" check.

[akpm@linux-foundation.org: reorganise code]
Signed-off-by: Jianguo Wu <wujianguo@huawei.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>