RAID 5/6 code merged into Btrfs

I probably should have written seek every disk. Metadata reads tend to be more seek intensive, and if every drive in the raid set needs to be involved to read every metadata block, you end up seek bound over the whole array pretty quickly.

We're only checking parity if we have to rebuild a block. The rebuilds only happen if the crc check fails, or if you get an IO error.

-chris