io_uring/zcrx: rename notif to event

io_uring/zcrx: rename notif to event

"Notification" is too long and the abbreviated version is used in
several places, which is inconsistent and more ambiguous for users.
Rename it to event, which is easier to keep consistent. To keep the
change small, only change uapi/ + do necessary fix ups, and the rest of
internals can be adjusted in the next release.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://patch.msgid.link/f95ca6717da3c8d3649a1a7f0d883a563f545052.1784726895.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>