md: ensure md devices are freed before module is unloaded.

Commit: cbd199837750 ("md: Fix unfortunate interaction with evms")
change mddev_put() so that it would not destroy an md device while
->ctime was non-zero.

Unfortunately, we didn't make sure to clear ->ctime when unloading
the module, so it is possible for an md device to remain after
module unload.  An attempt to open such a device will trigger
an invalid memory reference in:
  get_gendisk -> kobj_lookup -> exact_lock -> get_disk

when tring to access disk->fops, which was in the module that has
been removed.

So ensure we clear ->ctime in md_exit(), and explain how that is useful,
as it isn't immediately obvious when looking at the code.

Fixes: cbd199837750 ("md: Fix unfortunate interaction with evms")
Tested-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -8980,7 +8980,14 @@ static __exit void md_exit(void)
 
 	for_each_mddev(mddev, tmp) {
 		export_array(mddev);
+		mddev->ctime = 0;
 		mddev->hold_active = 0;
+		/*
+		 * for_each_mddev() will call mddev_put() at the end of each
+		 * iteration.  As the mddev is now fully clear, this will
+		 * schedule the mddev for destruction by a workqueue, and the
+		 * destroy_workqueue() below will wait for that to complete.
+		 */
 	}
 	destroy_workqueue(md_misc_wq);
 	destroy_workqueue(md_wq);