zebra: fix kernel-route's deletion on vrf

zebra: fix kernel-route's deletion on vrf

zebra can catch the kernel's route deletion by netlink.
but current FRR can't delete kernel-route on vrf(l3mdev)
when kernel operator delete the route on out-side of FRR.
It looks problem about kernel-route deletion.

This problem is caused around _nexthop_cmp_no_labels(nh1,nh2)
that checks the each nexthop's member 'vrf_id'.
And _nexthop_cmp_no_labels's caller doesn't set the vrf_id
of nexthop structure. This commit fix that case.

Signed-off-by: Hiroki Shirokura <slank.dev@gmail.com>