bcache: Set the logical block size to a sensible value

Preserve the backing device's logical size, use 512 byte
sectors when there is no backing device.

The logical block size has no impact on performance, but
it alters the meaning of on-disk structures like partition
tables.  Preserve the backing device's sector size to keep
bcache transparent, and use 512 byte sectors like everyone
else when no backing device is present.

Signed-off-by: Gabriel de Perthuis <g2p.code@gmail.com>
bcache: Refresh usage docs

Mention udev autoregistration, symlinks.  Write down some sysfs paths.