[RFC] epoll: avoid spinlock contention with wfcqueue

[Eric Wong]

I'm posting this lightly tested version since I may not be able to do
more testing/benchmarking until the weekend.

Davide's totalmess is still running, so that's probably a good sign :)
http://www.xmailserver.org/totalmess.c
I will look for more ways to break this (and benchmark when I stop
finding ways to break it).  No real applications tested, yet, and
I think I can improve upon this, too.

This depends on a couple of patches sitting in -mm and a few
more I've posted on LKML, for convenience everything is here:

	http://yhbt.net/epoll-wfcqueue-v3.8.2-20130314.mbox
(should apply cleanly to 3.9-rc* since there's no epoll changes in that)

--------------------------8<-------------------------------