io_uring: fix regbuf vector size truncation

io_uring: fix regbuf vector size truncation

There is a report of io_estimate_bvec_size() truncating the calculated
number of segments that leads to corruption issues. Check it doesn't
overflow "int"s used later. Rough but simple, can be improved on top.

Cc: stable@vger.kernel.org
Fixes: 9ef4cbbcb4ac3 ("io_uring: add infra for importing vectored reg buffers")
Reported-by: Google Big Sleep <big-sleep-vuln-reports+bigsleep-458654612@google.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Günther Noack <gnoack@google.com>
Tested-by: Günther Noack <gnoack@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>