Aleksei hit the soft lockup during reading /proc/PID/smaps.  David
investigated the problem and suggested the right fix.

while_each_thread() is racy and should die, this patch updates
vm_is_stack().
Signed-off-by: NOleg Nesterov <oleg@redhat.com>
Reported-by: NAleksei Besogonov <alex.besogonov@gmail.com>
Tested-by: NAleksei Besogonov <alex.besogonov@gmail.com>
Suggested-by: NDavid Rientjes <rientjes@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>