[#970] fstree: Move file locking to the generic writer

It is not a part of FSTree itself, but rather a way to solve concurrent
counter update on non-linux implementations. New linux implementations
is pretty simple: link fails when the file exists, unlink fails when the
file doesn't exist.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -53,9 +53,7 @@ type FSTree struct {
 	readOnly bool
 	metrics  Metrics
 
-	fileGuard          keyLock
-	fileCounter        FileCounter
-	fileCounterEnabled bool
+	fileCounter FileCounter
 
 	writer writer
 }
@@ -88,14 +86,13 @@ func New(opts ...Option) *FSTree {
 		Depth:       4,
 		DirNameLen:  DirNameLen,
 		metrics:     &noopMetrics{},
-		fileGuard:   &noopKeyLock{},
 		fileCounter: &noopCounter{},
 		log:         &logger.Logger{Logger: zap.L()},
 	}
 	for i := range opts {
 		opts[i](f)
 	}
-	f.writer = newGenericWriteData(f)
+	f.writer = newGenericWriteData(f.fileCounter, f.Permissions, f.noSync)
 
 	return f
 }
@@ -444,7 +441,7 @@ func (t *FSTree) GetRange(ctx context.Context, prm common.GetRangePrm) (common.G
 // initFileCounter walks the file tree rooted at FSTree's root,
 // counts total items count, inits counter and returns number of stored objects.
 func (t *FSTree) initFileCounter() error {
-	if !t.fileCounterEnabled {
+	if !counterEnabled(t.fileCounter) {
 		return nil
 	}