bgpd, zebra: Support NEXTHOP_IPV4_IFINDEX in nexthop_lookup api

bgpd, zebra: Support NEXTHOP_IPV4_IFINDEX in nexthop_lookup api

Since commit ba281d3d040, ospfd uses NEXTHOP_IPV4_IFINDEX
routes. The API between zebra and bgpd which is used to query
nexthops for recursive routes did not support this nexthop
type and therefore, ospf changes (or any other IGP changes
which use NEXTHOP_IPV4_IFINDEX) would never trigger any
recursive route update.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>