net/mlx5: refuse empty VLAN in flow pattern

net/mlx5: refuse empty VLAN in flow pattern

In verbs, an empty VLAN is equivalent to a packet without VLAN layer,
hence, the VLAN item should not be empty and this case is rejected.

However, the case for ether type of VLAN without following VLAN item
was not validated, allowing the creation of a flow with empty
VLAN item.

To fix this issue a validation was added requiring ether type of VLAN
will be followed with VLAN item.

Fixes: 0b1edd21cd78 ("net/mlx5: refuse empty VLAN flow specification")
Cc: stable@dpdk.org
Signed-off-by: Shiri Kuzin <shirik@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>