net/mlx5: fix selection between encap and decap

net/mlx5: fix selection between encap and decap

The current raw_encap function selects if to decap or encap
based on the egress/ingress attribute. This concepts doesn't work
in case of FDB since all flows are considered ingress.

To solve this issue we moved to check the encap size.
if the encap size is larger then eth + ipv4 it means we are
trying to encap.

Fixes: 8ba9eee4ce32 ("net/mlx5: add raw data encap/decap to Direct Verbs")
Cc: stable@dpdk.org
Signed-off-by: Ori Kam <orika@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>