zebra: fix detection of duplicate kernel routes (ECMP version)

zebra: fix detection of duplicate kernel routes (ECMP version)

Fixes the following bug:
% ip route add 50.0.0.0/8 nexthop via 10.0.1.2 nexthop via 10.0.2.2
% ip route replace 50.0.0.0/8 nexthop via 10.0.1.3 nexthop via 10.0.2.3
% ip route replace 50.0.0.0/8 nexthop via 10.0.1.4 nexthop via 10.0.2.4
%
% vtysh -c "show ip route"
[snip]
K * 50.0.0.0/8 [0/0] via 10.0.1.4, rt1-eth1, 00:00:00
  *                  via 10.0.2.4, rt1-eth2, 00:00:00
K * 50.0.0.0/8 [0/0] via 10.0.1.3, rt1-eth1, 00:00:10
  *                  via 10.0.2.3, rt1-eth2, 00:00:10
K>* 50.0.0.0/8 [0/0] via 10.0.1.2, rt1-eth1, 00:00:24
  *                  via 10.0.2.2, rt1-eth2, 00:00:24

Commit a3d18ce6 fixed a similar problem for single-path routes.

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