eventpoll: abstract out parameter sanity checking

Add a helper that checks the validity of the file descriptor and
other parameters passed in to epoll_wait().

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20250219172552.1565603-2-axboe@kernel.dk


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