nvmet: add req cns error complete helper

We report error and complete the request when identify cns value is not
handled in nvmet_execute_identify(). This error reporting is also needed
for Zone Block Device backend for NVMeOF target.

Add a helper nvmet_req_cns_error_compplete() to report an error and
complete the request when idenitfy command cns not handled value.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>