nvme: fix error handling in nvme_ns_report_zones

nvme_submit_sync_cmd can return positive NVMe error codes in addition to
the negative Linux error code, which are currently ignored.  Fix this
by removing __nvme_ns_report_zones and handling the errors from
nvme_submit_sync_cmd in the caller instead of multiplexing the return
value and the number of zones reported into a single return value.

Fixes: 240e6ee2 ("nvme: support for zoned namespaces")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>