fs/proc/task_mmu.c: shift "priv->task = NULL" from m_start() to m_stop()

1. There is no reason to reset ->tail_vma in m_start(), if we return
   IS_ERR_OR_NULL() it won't be used.

2. m_start() also clears priv->task to ensure that m_stop() won't use
   the stale pointer if we fail before get_task_struct(). But this is
   ugly and confusing, move this initialization in m_stop().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -146,17 +146,12 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	struct vm_area_struct *vma, *tail_vma = NULL;
 	loff_t l = *pos;
 
-	/* Clear the per syscall fields in priv */
-	priv->task = NULL;
-	priv->tail_vma = NULL;
-
 	/*
 	 * We remember last_addr rather than next_addr to hit with
 	 * vmacache most of the time. We have zero last_addr at
 	 * the beginning and also after lseek. We will have -1 last_addr
 	 * after the end of the vmas.
 	 */
-
 	if (last_addr == -1UL)
 		return NULL;
 
@@ -228,8 +223,10 @@ static void m_stop(struct seq_file *m, void *v)
 
 	if (!IS_ERR_OR_NULL(v))
 		vma_stop(priv);
-	if (priv->task)
+	if (priv->task) {
 		put_task_struct(priv->task);
+		priv->task = NULL;
+	}
 }
 
 static int proc_maps_open(struct inode *inode, struct file *file,