fuse: fix bug in wrappingFS, and add test coverage.

fuse/test/loopback_test.go

@@ -78,7 +78,7 @@ func NewTestCase(t *testing.T) *testCase {
 		})
 	me.connector.SetDebug(VerboseTest())
 	me.state, err = fuse.NewServer(
-		me.connector.RawFS(), me.mnt, &fuse.MountOptions{SingleThreaded: true})
+		fuse.NewRawFileSystem(me.connector.RawFS()), me.mnt, &fuse.MountOptions{SingleThreaded: true})
 	if err != nil {
 		t.Fatal("NewServer:", err)
 	}

fuse/upgrade.go

@@ -58,7 +58,6 @@ func (fs *wrappingFS) Forget(nodeID, nlookup uint64) {
 	}); ok {
 		s.Forget(nodeID, nlookup)
 	}
-	return
 }
 
 func (fs *wrappingFS) GetAttr(input *GetAttrIn, out *AttrOut) (code Status) {
@@ -76,7 +75,7 @@ func (fs *wrappingFS) Open(input *OpenIn, out *OpenOut) (status Status) {
 	}); ok {
 		return s.Open(input, out)
 	}
-	return OK
+	return ENOSYS
 }
 
 func (fs *wrappingFS) SetAttr(input *SetAttrIn, out *AttrOut) (code Status) {