Fix reverse gocryptfs.conf access on macOS

Unlike the FUSE implementation on Linux, macFUSE doesn't cache the file
attributes from the `LOOKUP` call, so it calls `GETATTR` prior to
accessing a file.

In the case of the `VirtualConfNode` (reverse config file passthrough),
this resulted in the default `GETATTR` implementation returning an empty
result, ultimately resulting in a "permission denied" error.

    14:44:14.095207 rx 3: GETATTR n2
    14:44:14.095229 tx 3:     OK, {tA=1s {M0100000 SZ=0 L=0 0:0 0 0:8954996 A 0.000000 M 0.000000 C 0.000000}}
    14:44:14.099943 rx 4: ACCESS n2 {u=501 g=20 r}
    14:44:14.099990 tx 4:     13=permission denied

By impementing `Getattr` (from `fs.NodeGetattrer`) on `VirtualConfNode`
this solves the issue.

internal/fusefrontend_reverse/virtualconf.go

@@ -10,6 +10,7 @@ import (
 )
 
 var _ = (fs.NodeOpener)((*VirtualConfNode)(nil))
+var _ = (fs.NodeGetattrer)((*VirtualConfNode)(nil))
 
 type VirtualConfNode struct {
 	fs.Inode
@@ -27,6 +28,17 @@ func (n *VirtualConfNode) Open(ctx context.Context, flags uint32) (fh fs.FileHan
 	return
 }
 
+func (n *VirtualConfNode) Getattr(ctx context.Context, fh fs.FileHandle, out *fuse.AttrOut) syscall.Errno {
+	var st syscall.Stat_t
+	err := syscall.Stat(n.path, &st)
+	if err != nil {
+		return fs.ToErrno(err)
+	}
+	out.FromStat(&st)
+	return 0
+}
+
+
 // Check that we have implemented the fs.File* interfaces
 var _ = (fs.FileReader)((*VirtualConfFile)(nil))
 var _ = (fs.FileReleaser)((*VirtualConfFile)(nil))