net/mlx5: fix layer validation with decapsulation

net/mlx5: fix layer validation with decapsulation

Currently, the flow validate function only validate the outermost layer
with the header modify actions. If there is decapsulation action before
the header modify action, the validation should choose the inner layer
for validation.

Add decapsulation check when validate with the header modify actions.
Choose the inner layer once there is decapsulation action.

Fixes: 4bb14c83df95 ("net/mlx5: support modify header using Direct Verbs")
Cc: stable@dpdk.org
Signed-off-by: Suanming Mou <suanmingm@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>