mempolicy: remove tasklist_lock from migrate_pages

Today, tasklist_lock in migrate_pages doesn't protect anything.
rcu_read_lock() provide enough protection from pid hash walk.

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempolicy.c

@@ -1310,16 +1310,13 @@ SYSCALL_DEFINE4(migrate_pages, pid_t, pid, unsigned long, maxnode,
 
 	/* Find the mm_struct */
 	rcu_read_lock();
-	read_lock(&tasklist_lock);
 	task = pid ? find_task_by_vpid(pid) : current;
 	if (!task) {
-		read_unlock(&tasklist_lock);
 		rcu_read_unlock();
 		err = -ESRCH;
 		goto out;
 	}
 	mm = get_task_mm(task);
-	read_unlock(&tasklist_lock);
 	rcu_read_unlock();
 
 	err = -EINVAL;