Device support for unmounting by block device is spotty

Either the kernel, or the userspace mount utility, used in TWRP for Google Pixel XL, does not properly support [unmounting by block device](https://github.com/dlenski/tetherback/blob/ac5400bf9a45294988520d5d92c81165939a19ee/tetherback/adb_stuff.py#L32).

```
$ adb -d shell 'grep vendor /proc/mounts'
/dev/block/bootdevice/by-name/vendor_b /vendor ext4 rw,seclabel,relatime,data=ordered 0 0

$ adb -d shell 'umount /dev/block/bootdevice/by-name/vendor_b'
umount: can't umount /dev/block/bootdevice/by-name/vendor_b: Invalid argument

$ adb -d shell 'umount /vendor'

$ adb -d shell 'grep vendor /proc/mounts'
```