RDMA/core: Allow drivers to disable restrack DB

RDMA/core: Allow drivers to disable restrack DB

Driver QP types are special case with no IBTA restrictions. For example,
EFA implemented creation of this QP type as regular one, while mlx5
separated create to two step: create and modify. That separation causes to
the situation where DC QP (mlx5) is always added to the same xarray index
zero.

This change allows to drivers like mlx5 simply disable restrack DB
tracking, but it doesn't disable kref on the memory.

Fixes: 52e0a118a203 ("RDMA/restrack: Track driver QP types in resource tracker")
Link: https://lore.kernel.org/r/20201117070148.1974114-3-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>