udev 60-block.rules does not watch bcache

[Scott Moser]

bcache devices do not have a watch on them from /lib/udev/rules.d/60-block.rules
The result is that this will fail:

  mkfs.ext4 /dev/bcache0 -L foobar
  udevadm settle
  ls -l /dev/disk/by-label/foobar

where as it will work on
  mkfs.ext4 /dev/vdb -L foobar
  udevadm settle
  ls -l /dev/disk/by-label/foobar

We found this when chasing a curtin bug (bug 1680591). The result of it was that
we were installing and an update-grub would choose to boot with root=/dev/bcache0
rather than UUID=.

Related bugs:
 * bug 1680591: installed system boots with root=/dev/bcache0 rather than root=UUID when root is bcache

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: udev 232-19
ProcVersionSignature: Ubuntu 4.9.0-15.16-generic 4.9.5
Uname: Linux 4.9.0-15-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.20.4-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity:Unity7
Date: Thu Apr 6 16:28:59 2017
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-07-23 (623 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.9.0-15-generic.efi.signed root=UUID=f897b32a-eacf-4191-9717-844918947069 ro quiet splash
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.vendor: Intel Corporation