fixup! wcfs: Handle ZODB invalidations

Fix last-minute error that crept in during
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.

wcfs/wcfs.go

@@ -954,7 +954,7 @@ retry:
 	for sk := range gdebug.zheadSockTab {
 		_, err := fmt.Fprintf(xio.BindCtxW(sk, ctx), "%s\n", δZ.Tid)
 		if err != nil {
-			log.Errorf("zhead: %s: write: %s  (detaching reader)", sk, err)
+			log.Errorf("zhead: %s: write: %s  (detaching reader)", sk.file, err)
 			sk.Close()
 			delete(gdebug.zheadSockTab, sk)
 		}