nvme: tuning pr code by using defined structs and macros

All the modifications are simply to make the code more readable,
and this patch does not include any functional changes.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>