nvme-pci: only call nvme_unmap_data for requests transferring data

This mirrors how nvme_map_pci is called and will allow simplifying some
checks in nvme_unmap_pci later on.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>