file extended attribute encodings

[jkominek]

Our KV layout gives us a lot of flexibility for encoding the inode data blocks. We've got basically a tiny amount of arbitrary data tacked at the ends of the keys, so that we can encode compression or parity information.

It'd be nice to be able to get the same thing going with xattr data, as well. Linux VFS allows attribute values up to 64kB, which is definitely a large enough lump of data to be worth compressing.

No immediate ideas for how to make that change.