nvmet: Reorder fields in 'struct nvmet_ns'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct nvmet_ns' from 520 to 512
bytes.

When such a structure is allocated in nvmet_ns_alloc(), because of the way
memory allocation works, when 520 bytes were requested, 1024 bytes were
allocated.

So, on x86_64, this change saves 512 bytes per allocation.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>