zebra: add an exception for link-local connected routes

zebra: add an exception for link-local connected routes

Link-local routes are unique in the sense that they all have the same
prefix but have different nexthops (local interfaces). Add an exception
in rib_add() to allows us to keep track of all of them.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>