selftests: mlxsw: Reduce router scale running time using offload indication

Currently, the test inserts X /32 routes and for each route it is
testing that a packet sent from the first host is received by the second
host, which is very time-consuming.

Instead only validate the offload flag of each route and get the same result.

Wait between the creation of the routes and the offload validation in
order to make sure that all the routes were successfully offloaded.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
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>