zebra: do not treat kernel routes as implicit withdraws

zebra: do not treat kernel routes as implicit withdraws

Kernel does not send the best route after adding or deleting routes, if
we treat routes for an existing prefix as implicit withdraw the zebra RIB
goes out of sync with FIB and can announce wrong route to protocols.

host:~# vtysh -c 'show ip route'
S>* 0.0.0.0/0 [0/0] via 192.168.1.1, eth0
C>* 192.168.1.0/24 is directly connected, eth0

host:~# ip route add 192.0.2.0/24 via 192.168.1.101 metric 100
host:~# vtysh -c 'show ip route'
S>* 0.0.0.0/0 [0/0] via 192.168.1.1, eth0
K>* 192.0.2.0/24 via 192.168.1.101, eth0
C>* 192.168.1.0/24 is directly connected, eth0

host:~# ip route add 192.0.2.0/24 via 192.168.1.102 metric 50
host:~# vtysh -c 'show ip route'
S>* 0.0.0.0/0 [0/0] via 192.168.1.1, eth0
K>* 192.0.2.0/24 via 192.168.1.102, eth0
C>* 192.168.1.0/24 is directly connected, eth0

host:~# ip route del 192.0.2.0/24 via 192.168.1.102 metric 50
host:~# vtysh -c 'show ip route'
S>* 0.0.0.0/0 [0/0] via 192.168.1.1, eth0
C>* 192.168.1.0/24 is directly connected, eth0
host:~# ip route show 192.0.2.0/24
192.0.2.0/24 via 10.10.1.101 dev eth0 metric 100

Signed-off-by: Jorge Boncompte <jbonor@gmail.com>