Convert more rtentry field accesses into nhop fields accesses.

Continue routing subsystem conversion to nhop objects defined in r359823.
Use fields from nhop structure instead of "struct rtentry" fields.
This is one of the last changes prior to removing rt_ifp, rt_ifa,
rt_gateway and rt_mtu from struct rtentry.

Differential Revision: https://reviews.freebsd.org/D24609