net/mlx5: fix VLAN pop with decap action validation

net/mlx5: fix VLAN pop with decap action validation

The combination of decap action followed by pop VLAN action is not
fully validated in existing code.

This patch updates the validation function of pop vlan action.
Pop VLAN with preceding Decap requires inner header with VLAN.
Pop VLAN without preceding Decap requires outer header with VLAN.

Fixes: b41e47da2592 ("net/mlx5: support pop flow action on VLAN header")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>