bcache block interface ongoing support

Thinking about it, you could do that in the dynamic state of the system with just a dm-linear that skips the start of the device. You'd definitely need something to get that set up though (with low enough requirements that it could run from an initramfs and link against musl etc). It would not exactly be entirely transparent, unless "bcache" remains as a block device that just does that and passes everything through, or that thunks to dm-cache for everything (now *that* would be neat, get any improvements with that layer without compatibility problems!).