Btrfs: set FMODE_EXCL in btrfs_device->mode

This fixes a bug introduced in d4d77629, where the device added online
(and therefore initialized via btrfs_init_new_device()) would be left
with the positive bdev->bd_holders after unmount.  Since d4d77629 we no
longer OR FMODE_EXCL explicitly on blkdev_put(), set it in
btrfs_device->mode.
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/volumes.c

@@ -1639,7 +1639,7 @@ int btrfs_init_new_device(struct btrfs_root *root, char *device_path)
 	device->dev_root = root->fs_info->dev_root;
 	device->bdev = bdev;
 	device->in_fs_metadata = 1;
-	device->mode = 0;
+	device->mode = FMODE_EXCL;
 	set_blocksize(device->bdev, 4096);
 
 	if (seeding_dev) {