Fix last-minute error that crept in during
kirr/wendelin.core@4af54da9 :

    (neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs$ go test
    # lab.nexedi.com/nexedi/wendelin.core/wcfs
    ./wcfs.go:957:4: Errorf format %s has arg sk of wrong type *lab.nexedi.com/nexedi/wendelin.core/wcfs.FileSock

Amends 4430de41.