io_uring: fix index calculation

When indexing into a provided buffer ring, do not subtract 1 from the
index.

Fixes: c7fb1942 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220613101157.3687-2-dylany@fb.com


Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>