nametransform: better error code on invalid diriv length
nametransform: better error code on invalid diriv length

go-fuse translates errors unknown to it into "function not
implemented", which is wrong in this case.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs,davesteele/gocryptfs,rfjakob/gocryptfs,davesteele/gocryptfs