net/mlx5: fix Tx when first segment size is too short

net/mlx5: fix Tx when first segment size is too short

First segment size must be at least 18 bytes, packets not respecting this
are silently not sent by the NIC but counted as sent by the PMD.  The only
way to figure out is compiling the PMD in debug mode.

Fixes: 6579c27c11a5 ("net/mlx5: remove gather loop on segments")
Cc: stable@dpdk.org
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>