Speedup Sender.ListFilesystems

It was getting placeholder state for every dataset, but it uses filter and
filter returns only non placeholder datasets. So every returned fs isn't
placeholder and it was exec `zfs get` for every datasets without reason.

internal/endpoint/endpoint.go

@@ -107,32 +107,13 @@ func (s *Sender) ListFilesystems(ctx context.Context) (*pdu.ListFilesystemRes,
 		return nil, err
 	}
 
-	rfss := make([]*pdu.Filesystem, 0, len(fss))
-	for _, a := range fss {
-		// TODO: dedup code with Receiver.ListFilesystems
-		l := getLogger(ctx).WithField("fs", a)
-		ph, err := zfs.ZFSGetFilesystemPlaceholderState(ctx, a)
-		if err != nil {
-			l.WithError(err).Error("error getting placeholder state")
-			return nil, fmt.Errorf(
-				"cannot get placeholder state for fs %q: %w", a, err)
+	rfss := make([]*pdu.Filesystem, len(fss))
+	for i, p := range fss {
+		rfss[i] = &pdu.Filesystem{
+			Path: p.ToString(),
+			// ResumeToken does not make sense from Sender. And IsPlaceholder too,
+			// because after FSFilter we got non plaseholder datasets only.
 		}
-		l.WithField("placeholder_state", fmt.Sprintf("%#v", ph)).
-			Debug("placeholder state")
-
-		if !ph.FSExists {
-			l.Error("inconsistent placeholder state: filesystem must exists")
-			err := fmt.Errorf(
-				"inconsistent placeholder state: filesystem %q must exist in this context",
-				a.ToString())
-			return nil, err
-		}
-
-		rfss = append(rfss, &pdu.Filesystem{
-			Path: a.ToString(),
-			// ResumeToken does not make sense from Sender
-			IsPlaceholder: ph.IsPlaceholder,
-		})
 	}
 
 	res := &pdu.ListFilesystemRes{Filesystems: rfss}