nvme-pci: fix freeing of the HMB descriptor table

The HMB descriptor table is sized to the maximum number of descriptors
that could be used for a given device, but __nvme_alloc_host_mem could
break out of the loop earlier on memory allocation failure and end up
using less descriptors than planned for, which leads to an incorrect
size passed to dma_free_coherent.

In practice this was not showing up because the number of descriptors
tends to be low and the dma coherent allocator always allocates and
frees at least a page.

Fixes: 87ad72a5 ("nvme-pci: implement host memory buffer support")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>