net/mlx5: fix modify header action position

net/mlx5: fix modify header action position

According to RTE flow the action order should be the order that the
actions were given.
In the case of modify actions the position of the action was always
last.

This commit solves this issue by saving the position of the first modify
action, and then adds to this position the pointer to the modify action.

Fixes: 4bb14c83df95 ("net/mlx5: support modify header using Direct Verbs")
Cc: stable@dpdk.org
Signed-off-by: Ori Kam <orika@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>