As for BCACHE_SB_VERSION_CDEV_WITH_UUID, can you explain why you added
that? I suspect it's needed but I can't remember why I didn't add it
when I added the new UUID format (or perhaps I just forgot)
I took the name from a comment in the kernel-side bcache.h.
BCACHE_SB_VERSION_CDEV is the version make-bcache writes, and
BCACHE_SB_VERSION_CDEV_WITH_UUID is what the kernel updates it too; I
just changed the version names so that user-side and kernel-side were
more consistent, internally and with each other.
The kernel doesn't discriminate these two versions when opening, so it
should be possible to define only the latter and deprecate the other.