DS: epoll: fix misordered EPOLL_CTL_DEL call

Any operations on an fd after POSIX::close() are invalid, so
epoll_ctl will fail.  Worse off, in a multi-threaded Perl, the
fd may be reused by another thread and EPOLL_CTL_DEL can hit the
wrong file description as a result.

cf. https://rt.cpan.org/Ticket/Display.html?id=129487