net/mlx5: fix L3 encapsulation flow validation

In order to configure L3 encapsulation\decapsulation flow to mlx5
devices, 2 actions should be added to the flow actions list:
RTE_FLOW_ACTION_TYPE_RAW_DECAP and RTE_FLOW_ACTION_TYPE_RAW_ENCAP.

One of the validations for this scenario is to check that modify actions
is not done before the L3 decapsulation, because it doesn't make sense
to decapsulate a modified data.

Wrongly, this check was done for the case of L3 encapsulation what
causes a validation failure in modify + L3 encapsulation flow.

Ignore this check in case of L3 encapsulation.

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