zebra: Add nh_vrf_id to 'struct route_entry`

zebra: Add nh_vrf_id to 'struct route_entry`

With VRF route-leaking we need to know what vrf
the nexthops are in compared to this vrf.  This
code adds the nh_vrf_id to the route entry and
sets it up correctly for the non-route-leaking
case.

The assumption here is that future commits
will make the nh_vrf_id *different* than
the vrf_id.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>