ovs-router: Fix flushing of local routes.

ovs-router: Fix flushing of local routes.

Since commit 8e4e45887ec3, priority of 'local' route entries no
longer matches with 'plen'.  This should be taken into account
while flushing cached routes, otherwise they will remain in OVS
even after removing them from the system:

  # ifconfig eth0 11.0.0.1
  # ovs-appctl ovs/route/show
    --- A new route synchronized from kernel route table ---
    Cached: 11.0.0.1/32 dev eth0 SRC 11.0.0.1 local
  # ifconfig eth0 0
  # ovs-appctl ovs/route/show
    -- the new route entry is still in ovs route table ---
    Cached: 11.0.0.1/32 dev eth0 SRC 11.0.0.1 local

CC: wenxu <wenxu@ucloud.cn>
Fixes: 8e4e45887ec3 ("ofproto-dpif-xlate: makes OVS native tunneling honor tunnel-specified source addresses")
Reported-by: Zheng Jingzhou <glovejmm@163.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-dev/2020-July/373093.html
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: William Tu <u9012063@gmail.com>