io_uring/eventfd: move trigger check into a helper

It's a bit hard to read what guards the triggering, move it into a
helper and add a comment explaining it too. This additionally moves
the ev_fd == NULL check in there as well.

Link: https://lore.kernel.org/r/20240921080307.185186-5-axboe@kernel.dk


Signed-off-by: Jens Axboe <axboe@kernel.dk>