vdpa/mlx5: Create direct MKEYs in parallel

Use the async interface to issue MTT MKEY creation.
Extra care is taken at the allocation of FW input commands
due to the MTT tables having variable sizes depending on
MR.

The indirect MKEY is still created synchronously at the
end as the direct MKEYs need to be filled in.

This makes create_user_mr() 3-5x faster, depending on
the size of the MR.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Message-Id: <20240830105838.2666587-3-dtatulea@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>