Comment 21 for bug 1729145

Untested patch. Hoping to convey the change that's needed.

- decouple emitting a cached_dev CHANGE uevent which includes dev.uuid and dev.label
   from bch_cached_dev_run() which only happens when a bcacheX device is bound to the
   actual backing block device (bcache0 -> vdb)

- update bch_cached_dev_run() to invoke bch_cached_dev_emit_change() as needed; no functional
  code path changes here

- Modify register_bcache to detect a re-registering of a bcache cached_dev, and in that case
  call bcache_cached_dev_emit_change() to emit the uevent needed to trigger SYMLINK generation