io_uring/net: don't skip notifs for failed requests

We currently only add a notification CQE when the send succeded, i.e.
cqe.res >= 0. However, it'd be more robust to do buffer notifications
for failed requests as well in case drivers decide do something fanky.

Always return a buffer notification after initial prep, don't hide it.
This behaviour is better aligned with documentation and the patch also
helps the userspace to respect it.

Cc: stable@vger.kernel.org # 6.0
Suggested-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9c8bead87b2b980fcec441b8faef52188b4a6588.1664292100.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>