nvmet: rename nvmet_execute_identify_cns_cs_ns

nvmet_execute_identify_ns_zns is a more descriptive name for the
function handling the "I/O Command Set Specific Identify Namespace
Data Structure for the Zoned Namespace Command Set".

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>