mlxsw: spectrum_router: Re-increase scale of IPv6 nexthop groups

As explained in commit fc25996e ("mlxsw: spectrum_router: Increase
scale of IPv6 nexthop groups"), each nexthop group is hashed by XOR-ing
the interface indexes of all the member nexthop devices.

To avoid many different nexthop groups ending up using the same key, the
above commit started hashing the interface indexes themselves before
they are XOR-ed.

However, in cases in which there are many nexthop groups that all use
the same nexthop device and only differ in the gateway IP, we can still
end up in a situation in which all the groups are using the same key.
This eventually leads to -EBUSY error from rhashtable during insertion.

Improve the situation by also making the gateway IP part of the key.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Alex Veber <alexve@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Tested-by: Alex Veber <alexve@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>