This normally doesn't cause any extra harm, but it does mean that we'll
increment the eventfd notification count, if one has been registered
with the ring. This can confuse applications, when they see more
notifications on the eventfd side than are available in the ring.

Do the nice thing and only increment this count, if we actually posted
(or even overflowed) events.
Reported-and-tested-by: NDan Melnic <dmm@fb.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>