nvme: make nvme_add_ns{_head}_cdev return void

nvme: make nvme_add_ns{_head}_cdev return void

The return code from nvme_add_ns_head_cdev() and nvme_add_ns_cdev() is
never checked, so make those functions return void.

A cdev add failure is tolerated during initialization, and flags
NVME_NS_CDEV_LIVE and NVME_NSHEAD_CDEV_LIVE are for determining whether
a cdev needs to be deleted during un-initialization.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>