vfs_getopt_pos
jfd_pos = 2 * vfs_getopt_pos(opts, "desc") + 1;
errmsg_pos = 2 * vfs_getopt_pos(opts, "errmsg") + 1;
errmsg_pos = vfs_getopt_pos(opts, "errmsg");
errmsg_pos = vfs_getopt_pos(optlist, "errmsg");
int vfs_getopt_pos(struct vfsoptlist *opts, const char *name);