nvme-pci: fix DMA direction of unmapping integrity data

DMA direction should be taken in dma_unmap_page() for unmapping integrity
data.

Fix this DMA direction, and reported in Guangwu's test.

Reported-by: Guangwu Zhang <guazhang@redhat.com>
Fixes: 4aedb705 ("nvme-pci: split metadata handling from nvme_map_data / nvme_unmap_data")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>