nvme-pci: fix error unwind in nvme_map_data

Properly unwind step by step using refactored helpers from nvme_unmap_data
to avoid a potential double dma_unmap on a mapping failure.

Fixes: 7fe07d14

 ("nvme-pci: merge nvme_free_iod into nvme_unmap_data")
Reported-by: Marc Orr <marcorr@google.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Marc Orr <marcorr@google.com>