io_uring/net: don't overflow multishot accept

Don't allow overflowing multishot accept CQEs, we want to limit
the grows of the overflow list.

Cc: stable@vger.kernel.org
Fixes: 4e86a2c9

 ("io_uring: implement multishot mode for accept")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7d0d749649244873772623dd7747966f516fe6e2.1691757663.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>