net/mlx5: add Linux-specific file with getter functions

net/mlx5: add Linux-specific file with getter functions

'ctx' type (field in 'struct mlx5_ctx_shared') is changed from 'struct
ibv_context *' to 'void *'.  'ctx' members which are verbs dependent
(e.g. device_name) will be accessed through getter functions which are
added to a new file under Linux directory: linux/mlx5_os.c.

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