Replace previous udev rules workaround with proper fix (LP: #1861941)

Good summary of the issue:

https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1861941/comments/27 https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1861941/comments/47 https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1861941/comments/56 https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1861941/comments/62

And this MR is the bcache-tools fix.

Note: Upstream isn't merging our requests for sometime now, but we are doing all tests at our side to make sure that, even if we have to carry this as a delta (from upstream) it works smoothly.

Thanks for considering this.