routing: simplify decompose_change_notification().

The function's goal is to compare old/new nhop/nexthop group for the route
and decompose it into the series of RTM_ADD/RTM_DELETE single-nhop
events, calling specified callback for each event.
Simplify it by properly leveraging the fact that both old/new groups
are sorted nhop-# ascending.

Tested by: Claudio Jeker<claudio.jeker@klarasystems.com>
Differential Revision: https://reviews.freebsd.org/D35598
MFC after: 2 weeks