Design

Don't need to get the inode and device set in getattr calls, so don't
need to worry about uniqueness; do need to track the other attributes.

Need to track three values for rs_decode: original_size, n, m; can
track original size as filesize * n - (small value), and encode
small_value.  To do proper erasure coding, also need to store a hash
of the encoded files because we need to verify a file before putting
it back together.  This is enough that it is probably worth storing it
all in the encoded files.  We cache verification of each file, and put
a version number in so if we improve the encoding, e.g. by chunked
hashes then we can be properly compatible.

Therefore the getattr operation can just stat the file in the import
dir, and if not there, try the same filepath in the various datadirs.