Bcache and filesystem recovery

It doesn't handle barriers just yet. That's coming soon, it's needed for write behind caching.

This isn't a correctness issue, if you don't have barriers you just have to wait for writes to complete before you start another to guarantee ordering. Barriers have historically taken awhile to add to things like raid, lvm and drbd, bcache is nothing special.

There is a bug in that if you're running bcache on something that supports barriers, bcache won't indicated that they're no longer supported... I haven't worried much about that since you can disable them in the filesystem and it's only till I get barriers implemented in bcache.