md: fix use after free when removing rdev via sysfs

rdev->mddev is no longer valid upon return from entry->store() when the
'remove' command is given.

Cc: <stable@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/md.c

@@ -2097,7 +2097,7 @@ rdev_attr_store(struct kobject *kobj, struct attribute *attr,
 			rv = -EBUSY;
 		else
 			rv = entry->store(rdev, page, length);
-		mddev_unlock(rdev->mddev);
+		mddev_unlock(mddev);
 	}
 	return rv;
 }