io_uring/net: refactor io_setup_async_addr

Instead of passing the right address into io_setup_async_addr() only
specify local on-stack storage and let the function infer where to grab
it from. It optimises out one local variable we have to deal with.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6bfa9ab810d776853eb26ed59301e2536c3a5471.1663668091.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>