An application can use the mere fact of epoll_wait returning an fd
as synchronization with the write on the fd that triggered the notification.
This pattern come up in an internal networking server (b/229276331).
If an fd is added to epoll, setup a link from the fd to the epoll fd
and use it for synchronization as well.
Reviewed By: melver
Differential Revision: https://reviews.llvm.org/D124518
16baf59c6d