nvmet: fix I/O Command Set specific Identify Controller

For an identify command with cns set to NVME_ID_CNS_CS_CTRL, the NVMe
2.0 specification states that:

If the I/O Command Set specified by the CSI field does not have an
Identify Controller data structure, then the controller shall return
a zero filled data structure. If the host requests a data structure for
an I/O Command Set that the controller does not support, the controller
shall abort the command with a status code of Invalid Field in Command.

However, the current implementation of this identify command in
nvmet_execute_identify() only handles the ZNS command set, returning an
error for the NVM command set, which is not compliant with the
specifications as we do support this command set.

Fix this by:
1) Renaming nvmet_execute_identify_cns_cs_ctrl() to
   nvmet_execute_identify_ctrl_zns() to continue handling the
   ZNS command set as is.
2) Introduce a nvmet_execute_identify_ctrl_ns() helper to handle the
   NVM command set, returning a zero filled nvme_id_ctrl_nvm data
   structure.
3) Modify nvmet_execute_identify() to call these helpers based on
   the csi specified, returning an error for unsupported command sets.

Fixes: aaf2e048 ("nvmet: add ZBD over ZNS backend support")
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Tested-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>