mlxsw: spectrum: Add hash table for IPv6 address mapping

The device supports forwarding entries such as routes and FDBs that
perform tunnel (e.g., VXLAN, IP-in-IP) encapsulation or decapsulation.
When the underlay is IPv6, these entries do not encode the 128 bit IPv6
address used for encapsulation / decapsulation. Instead, these entries
encode a 24 bit pointer to an array called KVDL where the IPv6 address
is stored.

Currently, only IP-in-IP with IPv6 underlay is supported, but subsequent
patches will add support for VxLAN with IPv6 underlay. To avoid
duplicating the logic required to store and retrieve these IPv6
addresses, introduce a hash table that will store the mapping between
IPv6 addresses and their KVDL index.

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>