By using the iod->nents field (the same way other I/O paths do), we can
avoid recalculating the number of sg entries at unmap time, and make
nvme_unmap_user_pages() easier to call.

Also, use the 'write' parameter instead of assuming DMA_FROM_DEVICE.
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>