net/mlx5: switch to detached VXLAN network devices

net/mlx5: switch to detached VXLAN network devices

Current design uses the VXLAN virtual devices attached
to outer network interface for decapsulation. Kernel
allows to use non-attached devices, so now we can create
not attached device and use it both for encapsulation
and decapsulation. Devices management becomes simpler,
less VXLAN devices are created and used.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>