mlxsw: spectrum_router: Increase scale of IPv6 nexthop groups

Unlike IPv4, the kernel does not consolidate IPv6 nexthop groups. To
avoid exhausting the device's adjacency table - where nexthops are
stored - the driver does this consolidation instead.

Each nexthop group is hashed by XOR-ing the interface indexes of all the
member nexthop devices. However, the ifindex itself is not hashed, which
can result in identical keys used for different groups and finally an
-EBUSY error from rhashtable due to too long objects list.

Improve the situation by hashing the ifindex itself.

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