iothread: fix epollfd leak in the process of delIOThread

iothread: fix epollfd leak in the process of delIOThread

When we call addIOThread, the epollfd created in aio_context_setup,
but not close it in the process of delIOThread, so the epollfd will leak.

Reorder the code in aio_epoll_disable and reuse it.

Signed-off-by: Jie Wang <[email protected]>
Message-Id: <1526517763[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
[Mention change to aio_epoll_disable in commit message. - Fam]
Signed-off-by: Fam Zheng <[email protected]>