nvme-pci: use blk_map_iter for p2p metadata

The dma_map_bvec helper doesn't work for p2p data, so use the same
blk_map_iter method that sgl uses for this memory type.

Reported-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>