md: fix /proc/mdstat refcounting

I have seen mdadm oops after successfully unloading md module.

This patch privents from unloading md module while
mdadm is polling /proc/mdstat.

Signed-off-by: Akinbou Mita <akinobu.mita@gmail.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>