net/mlx5_core: Export transport objects

To be used by mlx5_ib in the following patches for implementing
RAW PACKET QP.

Add mlx5_core_ prefix to alloc and delloc transport_domain since
they are exposed now.

Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>