nvme-multipath: system fails to create generic nvme device

NVME_NSHEAD_DISK_LIVE is a flag for struct nvme_ns_head, not nvme_ns.
The current code has a typo causing NVME_NSHEAD_DISK_LIVE never to
be cleared once device_add_disk_fails, causing the system never to
create the 'generic' character device. Even several rescan attempts
will change the situation and the system has to be rebooted to fix
the issue.

Fixes: 11384580 ("nvme-multipath: add error handling support for add_disk()")
Signed-off-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>