Implement MDB control path support, enabling the creation, deletion,
replacement and dumping of MDB entries in a similar fashion to the
bridge driver. Unlike the bridge driver, each entry stores a list of
remote VTEPs to which matched packets need to be replicated to and not a
list of bridge ports.

The motivating use case is the installation of MDB entries by a user
space control plane in response to received EVPN routes. As such, only
allow permanent MDB entries to be installed and do not implement
snooping functionality, avoiding a lot of unnecessary complexity.

Since entries can only be modified by user space under RTNL, use RTNL as
the write lock. Use RCU to ensure that MDB entries and remotes are not
freed while being accessed from the data path during transmission.

In terms of uAPI, reuse the existing MDB netlink interface, but add a
few new attributes to request and response messages:

* IP address of the destination VXLAN tunnel endpoint where the
  multicast receivers reside.

* UDP destination port number to use to connect to the remote VXLAN
  tunnel endpoint.

* VXLAN VNI Network Identifier to use to connect to the remote VXLAN
  tunnel endpoint. Required when Ingress Replication (IR) is used and
  the remote VTEP is not a member of originating broadcast domain
  (VLAN/VNI) [1].

* Source VNI Network Identifier the MDB entry belongs to. Used only when
  the VXLAN device is in external mode.

* Interface index of the outgoing interface to reach the remote VXLAN
  tunnel endpoint. This is required when the underlay destination IP is
  multicast (P2MP), as the multicast routing tables are not consulted.

All the new attributes are added under the 'MDBA_SET_ENTRY_ATTRS' nest
which is strictly validated by the bridge driver, thereby automatically
rejecting the new attributes.

[1] https://datatracker.ietf.org/doc/html/draft-ietf-bess-evpn-irb-mcast#section-3.2.2Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>