net/mlx5: lay groundwork for switch offloads

net/mlx5: lay groundwork for switch offloads

With mlx5, unlike normal flow rules implemented through Verbs for traffic
emitted and received by the application, those targeting different logical
ports of the device (VF representors for instance) are offloaded at the
switch level and must be configured through Netlink (TC interface).

This patch adds preliminary support to manage such flow rules through the
flow API (rte_flow).

Instead of rewriting tons of Netlink helpers and as previously suggested by
Stephen [1], this patch introduces a new dependency to libmnl [2]
(LGPL-2.1) when compiling mlx5.

[1] https://mails.dpdk.org/archives/dev/2018-March/092676.html
[2] https://netfilter.org/projects/libmnl/

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>