Update linux-core.c

This patch allows dup2 cloned fd to work with the event handler as intended.
The comment wasn't very elucidating to me, maybe you can rephrase it?  I think the situation you're trying to address here is:

1. This file descriptor has been watched before.
2. Something that wasn't libuv closed it.  The descriptor was removed from the epoll set.
3. Libuv still thinks it exists and tries to EPOLL_CTL_MOD its entry in the epoll set.
4. That fails with ENOENT because of 2 so try EPOLL_CTL_ADD.

That's right, isn't it?