[PATCHv2] nvmet: implement unique discovery NQN

[Sagi Grimberg]

>> Add a configfs attribute 'discovery_nqn' in the 'nvmet' configfs
>> directory to specify the unique discovery NQN.
> 
> That's a bit of a sparse commit log.  What do  unique discovery NQN do?
> Please include a reference and a bit of an explnation.  And tell us
> why this feature is useful for Linux to have.

I think that a unique NQN is better to authenticate against. That would
be the motivation in my mind here.