zebra: fix FIB route updates on *BSD

zebra: fix FIB route updates on *BSD

On *BSD, we update a route in the FIB by removing the old one and then
installing the new version.

With that said, on kernel_route_rib() we need to provide a pointer to
both the old version and the new version of the route.

We were, however, passing a pointer to the new version to both the
'old' and 'new' parameters. This is not a problem on Linux, which uses
NLM_F_REPLACE to update routes, but it breaks route updates on *BSD
because the 'old' parameter points to a route that is not installed in
the kernel. The kernel_route_rib() function then fails to uninstall the
supposedly 'old' route and can fail to install the new version as well if
the kernel doesn't support ECMP (e.g. FreeBSD with default configuration).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>