nvme.h: fixup ANA group descriptor format

ANA Phase 3 draft had the 'reserved' field in the group descriptor
format set to '23:17' (so that the first namespace identifier started
at byte 24), but that got move with the approved TP to '31:17'
(so that the first namespace identifier started at byte 32).

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>