nvme-pci: refactor nvme_unmap_data

Split out three helpers from nvme_unmap_data that will allow finer grained
unwinding from nvme_map_data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Marc Orr <marcorr@google.com>