nhrpd: replace nhrp route nexthop with onlink route when prefix=nh

nhrpd: replace nhrp route nexthop with onlink route when prefix=nh

There are cases where nhrp wants to create a nhrp route to gre interface
with the nexthop which is the same the prefix. This is the case with
ipv6:

ipv6 route a:ff::ff:4/128 via a:ff::ff:4:/128 dev gre1

This route entry is false from zebra point of view, and to avoid that,
the nexthop is ignored in nhrp only if the prefix equals the nexthop.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>