nvmet: return all zeroed buffer when we can't find an active namespace

Quote from Figure 106 in NVMe 1.3a:

  The Identify Namespace data structure is returned to the host for the
  namespace specified in the Namespace Identifier (CDW1.NSID) field if it
  is an active NSID. If the specified namespace is not an active NSID,
  then the controller returns a zero filled data structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@rimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>