MINOR: epoll: always initialize all of epoll_event to please valgrind

MINOR: epoll: always initialize all of epoll_event to please valgrind

valgrind complains that epoll_ctl() uses an epoll_event in which we
have only set the part we use from the data field (i.e. the fd). Tests
show that pre-initializing the struct in the stack doesn't have a
measurable impact so let's do it.