nvme-pci: move the call to nvme_cleanup_cmd out of nvme_unmap_data

Cleaning up the command setup isn't related to unmapping data, and
disentangling them will simplify error handling a bit down the road.

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>