md: check the return of mddev_find()

Check the return of mddev_find(), since it may fail due to out of
memeory or out of usable minor number.

The reason I chose -ENODEV instead of -ENOMEM or something else is
md_alloc() function chose that ;)
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -6618,6 +6618,9 @@ static int md_open(struct block_device *bdev, fmode_t mode)
 	struct mddev *mddev = mddev_find(bdev->bd_dev);
 	int err;
 
+	if (!mddev)
+		return -ENODEV;
+
 	if (mddev->gendisk != bdev->bd_disk) {
 		/* we are racing with mddev_put which is discarding this
 		 * bd_disk.