mlxsw: spectrum_router: Store nexthops in a hash table

Later in the patchset we'll add the NH_{ADD,DEL} events which will let
us know when a nexthop is considered to be dead. Based on these events
we need to be able to add or remove the nexthop from the device's
tables.

Therefore, store the private nexthop structs in a hash table and use the
kernel's fib_nh struct as the key, so that we'll be able to easily find
them when the events are received.

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>