zebra: fix resolving nexthop through itself

zebra: fix resolving nexthop through itself

Problems reported with zebra nht oscillating when a nexthop is resolved
using the same address to reach the  nexthop (for example, 10.0.0.8 is
resolved via 10.0.0.8/32.)  This fix removes this attempt to resolve
thru itself unless the route being resolved is also a host route.
This fix also walks up the tree looking for a less specific route to
reach the nexthop if needed.  Smoke testing completed successfully.

Ticket: CM-8192
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: CCR-6583
Testing done: Manual testing successful, bgp-min completed successfully
l3-smoke completed with two test changes required.