Allow Sender.ListFilesystems returns real placeholders faster

If filter contains one entry with recursive dataset

```yaml
datasets:
  - pattern: "foo/bar"
    recursive: true
```

we can reuse Receiver.ListFilesystems, which uses `zfs get` for getting list of
filsystems + needed properties.

So this commit restores original logic, but faster. See commit 9b91e31 for
reference.

internal/daemon/filters/entry.go

@@ -68,3 +68,10 @@ func (self *filterItem) CompatCompare(b *filterItem) int {
 	}
 	return cmp.Compare(self.path.Length(), b.path.Length())
 }
+
+func (self *filterItem) RecursiveDataset() *zfs.DatasetPath {
+	if !self.recursive || !self.mapping {
+		return nil
+	}
+	return self.path
+}

internal/daemon/filters/fsmapfilter.go

@@ -152,3 +152,10 @@ func (self *DatasetFilter) UserSpecifiedDatasets() zfs.UserSpecifiedDatasetsSet
 func (self *DatasetFilter) AsFilter() endpoint.FSFilter { return self }
 
 func (self *DatasetFilter) Empty() bool { return len(self.entries) == 0 }
+
+func (self *DatasetFilter) SingleRecursiveDataset() *zfs.DatasetPath {
+	if len(self.entries) != 1 {
+		return nil
+	}
+	return self.entries[0].RecursiveDataset()
+}

internal/endpoint/endpoint.go

@@ -96,6 +96,10 @@ func (s *Sender) filterCheckFS(fs string) (*zfs.DatasetPath, error) {
 func (s *Sender) ListFilesystems(ctx context.Context) (*pdu.ListFilesystemRes,
 	error,
 ) {
+	if root := s.FSFilter.SingleRecursiveDataset(); root != nil {
+		return listFilesystemsRecursive(ctx, root, zfs.PlaceholderPropertyName)
+	}
+
 	fss, err := zfs.ZFSListMapping(ctx, s.FSFilter)
 	if err != nil {
 		return nil, err
@@ -710,17 +714,25 @@ func (f subroot) MapToLocal(fs string) (*zfs.DatasetPath, error) {
 	return c, nil
 }
 
+func (f subroot) SingleRecursiveDataset() *zfs.DatasetPath {
+	return f.localRoot
+}
+
 const receiveResumeToken = "receive_resume_token"
 
 func (s *Receiver) ListFilesystems(ctx context.Context) (*pdu.ListFilesystemRes,
 	error,
 ) {
-	root := s.clientRootFromCtx(ctx)
+	return listFilesystemsRecursive(ctx, s.clientRootFromCtx(ctx),
+		zfs.PlaceholderPropertyName, receiveResumeToken)
+}
+
+func listFilesystemsRecursive(ctx context.Context, root *zfs.DatasetPath,
+	props ...string,
+) (*pdu.ListFilesystemRes, error) {
 	rootStr := root.ToString()
 	fsProps, err := zfs.ZFSGetRecursive(ctx, rootStr, -1,
-		[]string{"filesystem"},
-		[]string{zfs.PlaceholderPropertyName, receiveResumeToken},
-		zfs.SourceAny)
+		[]string{"filesystem"}, props, zfs.SourceAny)
 	if err != nil {
 		var errNotExist *zfs.DatasetDoesNotExist
 		if errors.As(err, &errNotExist) {

internal/zfs/mapping.go

@@ -14,13 +14,14 @@ type DatasetFilter interface {
 	// The caller owns the returned set.
 	// Implementations should return a copy.
 	UserSpecifiedDatasets() UserSpecifiedDatasetsSet
+	SingleRecursiveDataset() *DatasetPath
 }
 
 // A set of dataset names that the user specified in the configuration file.
 type UserSpecifiedDatasetsSet map[string]bool
 
 // Returns a DatasetFilter that does not filter (passes all paths)
-func NoFilter() DatasetFilter {
+func NoFilter() noFilter {
 	return noFilter{}
 }
 
@@ -36,6 +37,8 @@ func (noFilter) UserSpecifiedDatasets() UserSpecifiedDatasetsSet { return nil }
 
 func (noFilter) Empty() bool { return true }
 
+func (noFilter) SingleRecursiveDataset() *DatasetPath { return nil }
+
 func ZFSListMapping(ctx context.Context, filter DatasetFilter,
 ) ([]*DatasetPath, error) {
 	res, err := ZFSListMappingProperties(ctx, filter, nil)