mm/mempolicy.c: remove unnecessary nodemask check in kernel_migrate_pages()

1) task_nodes = cpuset_mems_allowed(current);
   -> cpuset_mems_allowed() guaranteed to return some non-empty
      subset of node_states[N_MEMORY].

2) nodes_and(*new, *new, task_nodes);
   -> after nodes_and(), the 'new' should be empty or appropriate
      nodemask(online node and with memory).

After 1) and 2), we could remove unnecessary check whether the 'new'
AND node_states[N_MEMORY] is empty.

Link: http://lkml.kernel.org/r/20190806023634.55356-1-wangkefeng.wang@huawei.comSigned-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1512,10 +1512,6 @@ static int kernel_migrate_pages(pid_t pid, unsigned long maxnode,
 	if (nodes_empty(*new))
 		goto out_put;
 
-	nodes_and(*new, *new, node_states[N_MEMORY]);
-	if (nodes_empty(*new))
-		goto out_put;
-
 	err = security_task_movememory(task);
 	if (err)
 		goto out_put;