RDMA/mthca: Combine special QP struct with mthca QP

As preparation for the removal of QP allocation logic, we need to ensure
that ib_core allocates the right amount of memory before a call to the
driver create_qp(). It requires from driver to have the same structs for
all types of QPs.

Link: https://lore.kernel.org/r/20200926102450.2966017-10-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>