CVE-2021-47040

In the Linux kernel, the following vulnerability has been resolved:

io_uring: fix overflows checks in provide buffers

Colin reported before possible overflow and sign extension problems in io_provide_buffers_prep(). As Linus pointed out previous attempt did nothing useful, see d81269fecb8ce ("io_uring: fix provide_buffers sign extension").

Do that with help of check__overflow helpers. And fix struct io_provide_buf::len type, as it doesn't make much sense to keep it signed.