Use ACCESS_RTE to guard **f_rte, use ACCESS_EATTRS to guard **f_eattrs.
Use f_rta_cow() before writing to rta or eattrs, use f_rte_cow() before
writing preference (stored in rte).

Do not access eattrs indirectly through (*f_rte)->attrs->eattrs, it is
way too slow. The cached pointer is faster.