[PATCH] nvmet: parametrize maximum number of namespaces

[Alex Talker]

 > Please no module param for this, right place is configfs subsys
 > attr.

That makes sense, I agree but I mainly aimed to start off with a simple 
change(replacing macro with load-time value, unchangeable at runtime).

So if I'd add this to /sys/kernel/configfs/nvmet/subsystems/<NQN> folder 
as, say attr_max_namespaces,
what would you propose on:
1. Storing the value? Would it make sense to specify it directly as NN 
or MNAN value or the like?
2. What to do if set value is lower than the current value? Abort if 
there's greater number of enabled namespaces than the specified value?

Best regards,
Alex