mlxsw: spectrum_ipip: Use common hash table for IPv6 address mapping

Use the common hash table introduced by the previous patch instead of
the IP-in-IP specific implementation.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>