coda_venus_readdir(): use offsetof()

	struct file *host_file;

	struct dentry *de;

	struct venus_dirent *vdir;

	unsigned long vdir_size =

	    (unsigned long)(&((struct venus_dirent *)0)->d_name);

	unsigned long vdir_size = offsetof(struct venus_dirent, d_name);

	unsigned int type;

	struct qstr name;

	ino_t ino;