Fix for epoll backend: use duplicated fd

[Corendos]

Description

While playing with cancellation on an old machine not supporting io_uring, I noticed that it was using the original fd, thus causing a error.FileDescriptorNotRegistered to appear.

Solution

Use the duplicated fd if the completion.flags.dup boolean is set, otherwise fallback to the completion.fd function.

[mitchellh]

Thanks!