get_task_mm: check PF_KTHREAD lockless

Nowadays PF_KTHREAD is sticky and it was never protected by ->alloc_lock. 
Move the PF_KTHREAD check outside of task_lock() section to make this code
more understandable.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

kernel/fork.c

@@ -1536,14 +1536,13 @@ struct mm_struct *get_task_mm(struct task_struct *task)
 {
 	struct mm_struct *mm;
 
+	if (task->flags & PF_KTHREAD)
+		return NULL;
+
 	task_lock(task);
 	mm = task->mm;
-	if (mm) {
-		if (task->flags & PF_KTHREAD)
-			mm = NULL;
-		else
-			mmget(mm);
-	}
+	if (mm)
+		mmget(mm);
 	task_unlock(task);
 	return mm;
 }