nvmet: reorder fields in 'struct nvmet_sq'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvmet_sq' from 472 to 464
bytes when CONFIG_NVME_TARGET_AUTH is defined.

This structure is embedded into some other structures, so it helps reducing
their sizes as well.

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>