RDMA/mlx5: Refactor transport domain bookkeeping logic

In preparation to enable loopback on a single user context move the logic
that enables/disables loopback to separate functions and group variables
under a single struct.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>