Linux epoll: Check both read and write kqueue events existence in EPOLL_CTL_ADD
Linux epoll EPOLL_CTL_ADD op handler should always check registration
of both EVFILT_READ and EVFILT_WRITE kevents to deceide if supplied
file descriptor fd is already registered with epoll instance.
Reviewed by: emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22515