net/mlx5: fix flow items size calculation

net/mlx5: fix flow items size calculation

flow_dv_get_item_len returns the actual header size of
an rte_flow item.

Changing any of the structs for rte_flow items by adding
or removing some extra fields will break this function.

This fixes the behavior by returning the actual header size
of each item.

Fixes: 34d41b7aa3bf ("net/mlx5: add VXLAN encap action to Direct Verbs")
Cc: stable@dpdk.org
Signed-off-by: Raslan Darawsheh <rasland@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>