io_uring: move max entry definition and ring sizing into header

In preparation for needing this somewhere else, move the definitions
for the maximum CQ and SQ ring size into io_uring.h. Make the
rings_size() helper available as well, and have it take just the setup
flags argument rather than the fill ring pointer. That's all that is
needed.

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