nvme: repack struct nvme_ns_head

ns_id, lba_shift and ms are always accessed for every read/write I/O in
nvme_setup_rw. By grouping these variables into one cacheline we can
safe some cycles.

4k sequential reads:

           baseline   patched
Bandwidth: 1620       1634
IOPs       66345579   66910939

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>