eventpoll: add epoll_sendevents() helper

Basic helper that copies ready events to the specified userspace
address. The event checking is quick and racy, it's up to the caller
to ensure it retries appropriately in case 0 events are copied.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20250219172552.1565603-4-axboe@kernel.dk


Signed-off-by: Christian Brauner <brauner@kernel.org>