io_uring: limit size of provided buffer ring

io_uring: limit size of provided buffer ring

The type of head and tail do not allow more than 2^15 entries in a
provided buffer ring, so do not allow this.
At 2^16 while each entry can be indexed, there is no way to
disambiguate full vs empty.

Signed-off-by: Dylan Yudaken <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Reviewed-by: Hao Xu <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>