The following patch changes the eventfd interface to de-couple the eventfd
memory context, from the file pointer instance.
Without such change, there is no clean way to racely free handle the
POLLHUP event sent when the last instance of the file* goes away.
Also, now the internal eventfd APIs are using the eventfd context instead
of the file*.
Another cleanup this patch does, is making AIO select EVENTFD, instead of
adding a bunch of empty function stubs inside eventfd.h.
Andrew, this better go via Avi and the KVM tree, since they have patches
that will be based on the new interface.