[PATCH] i915: slab shrinker have to return -1 if it cant shrink any objects

[KOSAKI Motohiro]

>> If you are thinking the shrinker protocol is too complicated, doc update
>> patch is really welcome.
> 
> Slab shrinkers have a nasty, crappy interface and the shrink_slab()
> code is full of bugs.  Rather that telling people to "update the
> documentation" because it's too complex, how about we fix the
> interface and the bugs?

If you can take your time for that, I don't hesitate to help you. ;)