mlxsw: spectrum_router: Add support for route replace

In case we got a replace event, then the replaced route must exist. If
the route isn't capable of multipath, then replace first matching
non-multipath capable route.

If the route is capable of multipath and matching multipath capable
route is found, then replace it. Otherwise, replace first matching
non-multipath capable route.

The new route is inserted before the replaced one. In case the replaced
route is currently offloaded, then it's overwritten in the device's table
by the new route and later deleted, thus not impacting routed traffic.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>