vdpa/mlx5: Use consistent RQT size

The current code evaluates RQT size based on the configured number of
virtqueues. This can raise an issue in the following scenario:

Assume MQ was negotiated.
1. mlx5_vdpa_set_map() gets called.
2. handle_ctrl_mq() is called setting cur_num_vqs to some value, lower
   than the configured max VQs.
3. A second set_map gets called, but now a smaller number of VQs is used
   to evaluate the size of the RQT.
4. handle_ctrl_mq() is called with a value larger than what the RQT can
   hold. This will emit errors and the driver state is compromised.

To fix this, we use a new field in struct mlx5_vdpa_net to hold the
required number of entries in the RQT. This value is evaluated in
mlx5_vdpa_set_driver_features() where we have the negotiated features
all set up.

In addition to that, we take into consideration the max capability of RQT
entries early when the device is added so we don't need to take consider
it when creating the RQT.

Last, we remove the use of mlx5_vdpa_max_qps() which just returns the
max_vas / 2 and make the code clearer.

Fixes: 52893733

 ("vdpa/mlx5: Add multiqueue support")
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>