mlxsw: spectrum_nve: Add API to resolve learned IP addresses

FDB notifications for entries learned from an NVE tunnel contain the IP
address of the remote VTEP. In the case of IPv4 underlay, the IP address
is specified as-is. IPv6 addresses on the other hand, are specified as
handles which then need to be used to query the actual address from the
device.

Only IPv4 underlay is currently supported, so we cannot receive
notifications for IPv6 addresses and therefore an error is returned when
one tries to resolve such an address.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>