zebra: Trust onlink flag for nexthop active resolution

zebra: Trust onlink flag for nexthop active resolution

When checking if a nexthop is active, if it has been marked as onlink,
just check on the presence and status of the nexthop's interface. When
handling client request to create a route, if the client says that the
nexthop is onlink, trust it; when internally (in zebra) determining
that the nexthop is onlink, ensure it is only done in the case of an
interface with a /32 IP address which is the case for OSPF unnumbered.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by:   Stephen Worley <sworley@cumulusnetworks.com>