nvme: add reservation command's defines

This is a preparation patch for NVMeOF target reservation
commands implantation.
Add the defines of reservation command, such as reservation log
and sub operations.

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