mlxsw: spectrum_nve: Keep track of IPv6 addresses used by FDB entries

FDB entries that perform VxLAN encapsulation with an IPv6 underlay hold
a reference on a resource. Namely, the KVDL entry where the IPv6
underlay destination IP is stored. When such an FDB entry is deleted, it
needs to drop the reference from the corresponding KVDL entry.

To that end, maintain a hash table that maps an FDB entry (i.e., {MAC,
FID}) to the IPv6 address used by it.

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>