net/mlx5: refactor multi-process communication

net/mlx5: refactor multi-process communication

1. The shared data communication between the primary and the secondary
processes is implemented using Linux API. Move the Linux API code under
linux directory (file linux/mlx5_os.c).

2. File net/mlx5/mlx5_mp.c handles requests to the primary and secondary
processes (e.g. start_rxtx, stop_rxtx). It is Linux based so it is moved
under linux (new file linux/mlx5_mp_os.c).

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>