io_uring: fix types in provided buffer ring

io_uring: fix types in provided buffer ring

The type of head needs to match that of tail in order for rollover and
comparisons to work correctly.

Without this change the comparison of tail to head might incorrectly allow
io_uring to use a buffer that userspace had not given it.

Fixes: c7fb19428d67 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220613101157.3687-3-dylany@fb.com
Reviewed-by: Hao Xu <howeyxu@tencent.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>