get rid of ep_push_nested()

The only remaining user is loop checking.  But there we only need
to check that we have not walked into the epoll we are inserting
into - we are adding an edge to acyclic graph, so any loop being
created will have to pass through the source of that edge.

So we don't need that array of cookies - we have only one eventpoll
to watch out for.  RIP ep_push_nested(), along with the cookies
array.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>