Simplify nhop operations in ip_output().

Consistently use nh instead of always dereferencing
ro->ro_nh inside the if block.
Always use nexthop mtu, as it provides guarantee that mtu is accurate.
Pass nh pointer to rt_update_ro_flags() to allow upcoming uses
of updating ro flags based on different nexthop.

Differential Revision: https://reviews.freebsd.org/D31451
Reviewed by: kp
MFC after: 2 weeks