nvme: reorganize nvme_ns_head fields

shuffle few fields to reduce the holes within nvme_ns_head.
On x86_64, the size is reduced to 1104 bytes from 1120 bytes.

Signed-off-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>