mlxsw: Add support for VxLAN with IPv6 underlay

Currently, mlxsw driver supports VxLAN with IPv4 underlay only.
Add support for IPv6 underlay.

The main differences are:

* Learning is not supported for IPv6 FDB entries, use static entries and
  do not allow 'learning' flag for IPv6 VxLAN.

* IPv6 addresses for FDB entries should be saved as part of KVDL.
  Use the new API to allocate and release entries for IPv6 addresses.

* Spectrum ASICs do not fill UDP checksum, while in software IPv6 UDP
  packets with checksum zero are dropped.
  Force the relevant flags which allow the VxLAN device to generate UDP
  packets with zero checksum and also receive them.

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>