fixed rare panic

pkg/fs/metadata.go

@@ -362,6 +362,10 @@ type Metadata struct {
 func (md *Metadata) Close() (err error) {
 	md.mu.Lock()
 	defer md.mu.Unlock()
+	return md.close()
+}
+
+func (md *Metadata) close() (err error) {
 	if md.wc != nil {
 		close(md.wc)
 		if md.done != nil {
@@ -383,9 +387,7 @@ func (md *Metadata) Delete() error {
 	md.cmu.Lock()
 	md.Chunks = chunk.Chunks{}
 	md.cmu.Unlock()
-	close(md.wc)
-	md.f.Close()
-	md.f = nil
+	md.close()
 	atomic.StoreInt64(&md.Atime, now())
 	return md.fs.RemoveDst(md.name)
 }