io_uring/eventfd: dedup signalling helpers

Consolidate io_eventfd_flush_signal() and io_eventfd_signal(). Not much
of a difference for now, but it prepares it for following changes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/5beecd4da65d8d2d83df499196f84b329387f6a2.1745493845.git.asml.silence@gmail.com


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