net/mlx5: install a socket to exchange a file descriptor

net/mlx5: install a socket to exchange a file descriptor

Use a unix socket to get back the communication channel with the Kernel
driver from the primary process, this is necessary to remap those pages
in the secondary process memory space and thus use the same Tx queues.

This is only supported from rdma-core (v15).

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Signed-off-by: Xueming Li <xuemingl@mellanox.com>