nvmet: make discovery NQN configurable

TPAR8013 allows for unique discovery NQNs, so make the discovery
controller NQN configurable by exposing a subsys attribute
'discovery_nqn'.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>