io_uring: don't fail submit with overflow backlog

Don't fail submission attempts if there are CQEs in the overflow
backlog, but give away the decision making to the userspace. It
might be very inconvenient to the userspace, especially if
submission and completion are done by different threads.

We can remove it because of recent changes, where requests
are now not locked by the backlog, backlog entries are allocated
separately, so they take less space and cgroup accounted.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>