vxlan: Export address checking functions

Drivers that support VxLAN offload need to be able to sanitize the
configuration of the VxLAN device and accept / reject its offload.

For example, mlxsw requires that the local IP of the VxLAN device be set
and that packets be flooded to unicast IP(s) and not to a multicast
group.

Expose the functions that perform such checks.

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