[#479] writecache: Fix writecache fstree flush premature ctx cancel

Signed-off-by: Alejandro Lopez <a.lopez@yadro.com>

pkg/local_object_storage/writecache/flush.go

@@ -37,7 +37,6 @@ const (
 // runFlushLoop starts background workers which periodically flush objects to the blobstor.
 func (c *cache) runFlushLoop() {
 	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
 
 	ch := c.closeCh
 	c.wg.Add(1)