net/mlx4: Set proper build dependancy with vxlan

Make sure that vxlan_get_rx_port() is present in the kernel build in a manner
consistent with mlx4, else mlx4 can be made built-in where vxlan a module and
the phase of the build linking fails. Add CONFIG_MLX4_EN_VXLAN for that.

Also, #ifdef the advertizement and implementation of the mlx4 vxlan ndo
calls and related code under this config directive.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>