Discard should be whitelisted for bcache devices

Discard should be whitelisted for bcache devices

Currently should_enable_discard checks to see if we should enable discard based on the SATA version number. Unless "smartctl" works on the device and returns that the device is SATA 3.0 or newer then it will always disable discard - with exceptions added for /dev/nvme* and /dev/vd* devices.

We should also add exceptions for bcache devices, as bcache supports Discard and will prune data in the cache that hasn't yet been written back to the backing disks and prevent the no longer required data from being written.

This is a critical optimization since bluestore is Copy-On-Write. If data is changed its written to a new area of the disk each time - so the same data being overwritten multiple times will get written back to the backing disk multiple times without Discard enabled.