net/mlx5: support metadata as flow rule criteria

net/mlx5: support metadata as flow rule criteria

As described in series starting at [1], it adds option to set
metadata value as match pattern when creating a new flow rule.

This patch adds metadata support in mlx5 driver, in two parts:
- Add the validation and setting of metadata value in matcher,
  when creating a new flow rule.
- Add the passing of metadata value from mbuf to wqe when
  indicated by ol_flag, in different burst functions.

[1] "ethdev: support metadata as flow rule criteria"
    http://mails.dpdk.org/archives/dev/2018-September/113269.html

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>