EPOLL_CTL_DISABLE and multithreaded applications

Other solution: pass a pointer to a userspace reference count in a new EPOLL_CTL_ADD_RC, which is incremented in the kernel under the epoll lock when an event concerning that fd is returned to userspace.

This way, after EPOLL_CTL_DEL either the fd will never be returned or the reference count has been raised already.

Userspace just needs to be changed to use EPOLL_CTL_ADD_RC and to decrement the reference count after it finished processing the event, and delete the fd data if it goes to zero either at that point or after EPOLL_CTL_DEL.