Rename rt_flags to rte_flags && reduce number of rt_nhop accesses.

No functional changes.

Most of the routing flags are stored in the netxtop instead of rtentry.
Rename rt->rt_flags to rt->rte_flags to simplify reading/modifying code
checking routing flags.

In the new multipath code, rt->rt_nhop may actually point to nexthop group
instead of nhop. To ease transition, reduce the amount of rt->rt_nhop->...
accesses.

Differential Revision: https://reviews.freebsd.org/D26156