selftests/net: return back io_uring zc send tests

Enable io_uring zerocopy send tests back and fix them up to follow the
new inteface.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c8e5018c516093bdad0b6e19f2f9847dea17e4d2.1662027856.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>