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 d81269fe

 ("io_uring: fix provide_buffers sign extension").

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

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: efe68c1c

 ("io_uring: validate the full range of provided buffers for access")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/46538827e70fce5f6cdb50897cff4cacc490f380.1618488258.git.asml.silence@gmail.com


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