fs/proc/task_nommu.c: don't use priv->task->mm

I do not know if CONFIG_PREEMPT/SMP is possible without CONFIG_MMU
but the usage of task->mm in m_stop(). The task can exit/exec before
we take mmap_sem, in this case m_stop() can hit NULL or unlock the
wrong rw_semaphore.

Also, this code uses priv->task != NULL to decide whether we need
up_read/mmput. This is correct, but we will probably kill priv->task.
Change m_start/m_stop to rely on IS_ERR_OR_NULL() like task_mmu.c does.

Signed-off-by: Oleg Nesterov <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Cyrill Gorcunov <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Acked-by: Greg Ungerer <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/proc/task_nommu.c

···
 		return ERR_PTR(-ESRCH);
 
 	mm = priv->mm;
-	if (!mm || !atomic_inc_not_zero(&mm->mm_users)) {
-		put_task_struct(priv->task);
-		priv->task = NULL;
+	if (!mm || !atomic_inc_not_zero(&mm->mm_users))
 		return NULL;
-	}
-	down_read(&mm->mmap_sem);
 
+	down_read(&mm->mmap_sem);
 	/* start from the Nth VMA */
 	for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
 		if (n-- == 0)
 			return p;
+
+	up_read(&mm->mmap_sem);
+	mmput(mm);
 	return NULL;
 }
 
···
 {
 	struct proc_maps_private *priv = m->private;
 
+	if (!IS_ERR_OR_NULL(_vml)) {
+		up_read(&priv->mm->mmap_sem);
+		mmput(priv->mm);
+	}
 	if (priv->task) {
-		struct mm_struct *mm = priv->task->mm;
-		up_read(&mm->mmap_sem);
-		mmput(mm);
 		put_task_struct(priv->task);
+		priv->task = NULL;
 	}
 }