nvme-pci: merge nvme_free_iod into nvme_unmap_data

This means we now have a function that undoes everything nvme_map_data
does and we can simplify the error handling a bit.

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>