fusefrontend: drop last remaining call into loopbackFileSystem

The only call forwarded to loopbackFileSystem was Statfs,
which is trivial to implement.

Implement it and drop loopbackFileSystem, as having it carries the
risk that a coding error bypasses the usual encryption/decryption
chain.

internal/fusefrontend/fs.go

@@ -25,8 +25,11 @@ import (
 
 // FS implements the go-fuse virtual filesystem interface.
 type FS struct {
-	pathfs.FileSystem      // loopbackFileSystem, see go-fuse/fuse/pathfs/loopback.go
-	args              Args // Stores configuration arguments
+	// Embed pathfs.defaultFileSystem to avoid compile failure when the
+	// pathfs.FileSystem interface gets new functions. defaultFileSystem
+	// provides a no-op implementation for all functions.
+	pathfs.FileSystem
+	args Args // Stores configuration arguments
 	// dirIVLock: Lock()ed if any "gocryptfs.diriv" file is modified
 	// Readers must RLock() it to prevent them from seeing intermediate
 	// states
@@ -56,7 +59,7 @@ type FS struct {
 	dirCache dirCacheStruct
 }
 
-var _ pathfs.FileSystem = &FS{} // Verify that interface is implemented.
+//var _ pathfs.FileSystem = &FS{} // Verify that interface is implemented.
 
 // NewFS returns a new encrypted FUSE overlay filesystem.
 func NewFS(args Args, c *contentenc.ContentEnc, n *nametransform.NameTransform) *FS {
@@ -67,7 +70,7 @@ func NewFS(args Args, c *contentenc.ContentEnc, n *nametransform.NameTransform)
 		tlog.Warn.Printf("Forward mode does not support -exclude")
 	}
 	return &FS{
-		FileSystem:    pathfs.NewLoopbackFileSystem(args.Cipherdir),
+		FileSystem:    pathfs.NewDefaultFileSystem(),
 		args:          args,
 		nameTransform: n,
 		contentEnc:    c,
@@ -381,7 +384,14 @@ func (fs *FS) Utimens(path string, a *time.Time, m *time.Time, context *fuse.Con
 //
 // Symlink-safe because the passed path is ignored.
 func (fs *FS) StatFs(path string) *fuse.StatfsOut {
-	return fs.FileSystem.StatFs("")
+	var st syscall.Statfs_t
+	err := syscall.Statfs(fs.args.Cipherdir, &st)
+	if err == nil {
+		var out fuse.StatfsOut
+		out.FromStatfsT(&st)
+		return &out
+	}
+	return nil
 }
 
 // decryptSymlinkTarget: "cData64" is base64-decoded and decrypted