use exact allocation for dma coherent memory

[Christoph Hellwig]

> drivers/infiniband/hw/cxgb4/qp.c
>    129  static int alloc_host_sq(struct c4iw_rdev *rdev, struct t4_sq *sq)
>    130  {
>    131          sq->queue = dma_alloc_coherent(&(rdev->lldi.pdev->dev), sq->memsize,
>    132                                         &(sq->dma_addr), GFP_KERNEL);
>    133          if (!sq->queue)
>    134                  return -ENOMEM;
>    135          sq->phys_addr = virt_to_phys(sq->queue);
>    136          dma_unmap_addr_set(sq, mapping, sq->dma_addr);
>    137          return 0;
>    138  }
> 
> Is this a bug?

Yes.  This will blow up badly on many platforms, as sq->queue
might be vmapped, ioremapped, come from a pool without page backing.