mlxsw: spectrum_router: Align with new route replace logic

Commit f34436a4 ("net/ipv6: Simplify route replace and appending
into multipath route") changed the IPv6 route replace logic so that the
first matching route (i.e., same metric) is replaced.

Have mlxsw replace the first matching route as well.

Fixes: f34436a4

 ("net/ipv6: Simplify route replace and appending into multipath route")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>