nvme: add transport SGL definitions

Add transport SGL defintions from NVMe TP 4008, required for
the final NVMe-FC standard.

Signed-off-by: James Smart <james.smart@broadcom.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>