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>

drivers/md/md.c

@@ -4325,6 +4325,7 @@ static unsigned int mdstat_poll(struct file *filp, poll_table *wait)
 }
 
 static struct file_operations md_seq_fops = {
+	.owner		= THIS_MODULE,
 	.open           = md_seq_open,
 	.read           = seq_read,
 	.llseek         = seq_lseek,