nvme-pci: don't allocate dma_vec for IOVA mappings

Not only do IOVA mappings no need the separate dma_vec tracking, it
also won't free it and thus leak the allocations.

Fixes: b8b7570a ("nvme-pci: fix dma unmapping when using PRPs and not using the IOVA mapping")
Reported-by: Klara Modin <klarasmodin@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Klara Modin <klarasmodin@gmail.com>
Link: https://lore.kernel.org/r/20250711112250.633269-1-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>