fs: return ENOTSUP if NodeMknoder isn't implemented

This avoids a panic.

Change-Id: I5c4cc0d875ecdbc3b07c2322b5a5a5e6fa493709

fs/bridge.go

@@ -418,6 +418,8 @@ func (b *rawBridge) Mknod(cancel <-chan struct{}, input *fuse.MknodIn, name stri
 	var errno syscall.Errno
 	if mops, ok := parent.ops.(NodeMknoder); ok {
 		child, errno = mops.Mknod(&fuse.Context{Caller: input.Caller, Cancel: cancel}, name, input.Mode, input.Rdev, out)
+	} else {
+		return fuse.ENOTSUP
 	}
 
 	if errno != 0 {

fs/simple_test.go

@@ -356,6 +356,24 @@ func TestMknod(t *testing.T) {
 	}
 }
 
+func TestMknodNotSupported(t *testing.T) {
+	mountPoint := testutil.TempDir()
+	defer os.Remove(mountPoint)
+
+	server, err := Mount(mountPoint, &Inode{}, nil)
+	if err != nil {
+		t.Fatalf("cannot mount: %v", err)
+	}
+
+	defer server.Unmount()
+
+	name := filepath.Join(mountPoint, "foo")
+
+	if got, want := syscall.Mknod(name, syscall.S_IFREG|0755, (8<<8)|0), syscall.ENOTSUP; got != want {
+		t.Fatalf("mknod: got %v, want %v", got, want)
+	}
+}
+
 func TestPosix(t *testing.T) {
 	noisy := map[string]bool{
 		"ParallelFileOpen": true,