net/mlx5: fix GTP handling in header modify action

net/mlx5: fix GTP handling in header modify action

GTP items were ignored during conversion of modify header actions. This
caused modify TTL action to generate a wrong modify header command when
tunnel and inner headers used different IP versions.

This patch adds GTP item handling to modify header action conversion.

Fixes: 04233f36c712 ("net/mlx5: fix layer type in header modify action")
Cc: stable@dpdk.org
Signed-off-by: Dariusz Sosnowski <dsosnowski@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>