zebra: Abstract `dplane_ctx_route_init` to init route without copying

zebra: Abstract `dplane_ctx_route_init` to init route without copying

The function `dplane_ctx_route_init` initializes a dplane route context
from the route object passed as an argument. Let's abstract this
function to allow initializing the dplane route context without actually
copying a route object.

This allows us to use this function for initializing a dplane route
context when we don't have any route to copy in it.

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>
(cherry picked from commit 745a0fcbb21a6bd7330454437abe34aff0bf82c0)