proc: fix oops on invalid /proc/<pid>/maps access

When m_start returns an error, the seq_file logic will still call m_stop
with that error entry, so we'd better make sure that we check it before
using it as a vma.

Introduced by commit ec6fd8a ("report errors in /proc/*/*map*
sanely"), which replaced NULL with various ERR_PTR() cases.

(On ia64, you happen to get a unaligned fault instead of a page fault,
since the address used is generally some random error code like -EPERM)

Reported-by: Anca Emanuel <anca.emanuel@gmail.com>
Reported-by: Tony Luck <tony.luck@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Américo Wang <xiyou.wangcong@gmail.com>
Cc: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -182,7 +182,8 @@ static void m_stop(struct seq_file *m, void *v)
 	struct proc_maps_private *priv = m->private;
 	struct vm_area_struct *vma = v;
 
-	vma_stop(priv, vma);
+	if (!IS_ERR(vma))
+		vma_stop(priv, vma);
 	if (priv->task)
 		put_task_struct(priv->task);
 }