bgpd: prevent routes loop through itself

bgpd: prevent routes loop through itself

Some BGP updates received by BGP invite local router to
install a route through itself. The system will not do it, and
the route should be considered as not valid at the earliest.

This case is detected on the zebra, and this detection prevents
from trying to install this route to the local system. However,
the nexthop tracking mechanism is called, and acts as if the route
was valid, which is not the case.

By detecting in BGP that use case, we avoid installing the invalid
routes.

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