mount: fix caching of old directories after renaming them

It was discovered `rclone mount` (but not `rclone cmount`) cached
directories after rename which it shouldn't have done.

This caused IO errors when trying to access files in renamed
directories on bucket based file systems.

This turned out to be the kernel caching the directories as basil/fuse
sets their expiry time to 60s for some reason.

This fix invalidates the relevant kernel cache entries in the for the
directories which fixes the problem.

Fixes: #4977
See: https://forum.rclone.org/t/after-a-directory-renmane-using-mv-files-are-not-visible-any-longer/22797

cmd/mount/mount.go

@@ -91,12 +91,12 @@ func mount(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (<-chan error
 	}
 
 	filesys := NewFS(VFS, opt)
-	server := fusefs.New(c, nil)
+	filesys.server = fusefs.New(c, nil)
 
 	// Serve the mount point in the background returning error to errChan
 	errChan := make(chan error, 1)
 	go func() {
-		err := server.Serve(filesys)
+		err := filesys.server.Serve(filesys)
 		closeErr := c.Close()
 		if err == nil {
 			err = closeErr