net/mlx5: fix layer type in header modify action

net/mlx5: fix layer type in header modify action

Currently, for header modify actions after tunnel decapsulation, the
header modify actions will still select the old outermost layer type.
It will cause header modify actions get the wrong layer type.

Add the tunnel decap flag to indicate the layer type to get for header
modify actions.

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>