NFS: Replace readdir's use of xxhash() with hash_64()

Both xxhash() and hash_64() appear to give similarly low collision
rates with a standard linearly increasing readdir offset. They both give
similarly higher collision rates when applied to ext4's offsets.

So switch to using the standard hash_64().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>