nvmet: reorder fields in 'struct nvme_dhchap_queue_context'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvme_dhchap_queue_context' from
416 to 400 bytes.

This structure is kvcalloc()'ed in nvme_auth_init_ctrl(), so it is likely
that the allocation can be relatively big. Saving 16 bytes per structure
may might a slight difference.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>