nvme-pci: convert to blk_rq_dma_map

nvme-pci: convert to blk_rq_dma_map

Use the blk_rq_dma_map API to DMA map requests instead of
scatterlists.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Jens Axboe <axboe@kernel.dk>
[ Leon: squashed optimization patch from Kanchan ]
Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
[ Leon: rewrote original patch due to rebases and addition of metadata support ]
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>