io_uring: fix using under-expanded iters

[ upstream commit cd658695 ]

The issue was first described and addressed in
89c2b3b7 ("io_uring: reexpand under-reexpanded iters"), but
shortly after reimplemented as.
cd658695 ("io_uring: use iov_iter state save/restore helpers").

Here we follow the approach from the second patch but without in-callback
resubmissions, fixups for not yet supported in 5.10 short read retries
and replacing iov_iter_state with iter copies to not pull even more
dependencies, and because it's just much simpler.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>