When using HMB the PCIe host driver allocates host_mem_desc_bufs using
dma_alloc_attrs() but frees them using dma_free_coherent(). Use the
correct dma_free_attrs() function to free the buffers.

Signed-off-by: Liviu Dudau <liviu@dudau.co.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>