io_uring: reshuffle io_uring/io_uring.h

It's a good idea to first do forward declarations and then inline
helpers, otherwise there will be keep stumbling on dependencies
between them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/1d7fa6672ed43f20ccc0c54ae201369ebc3ebfab.1655637157.git.asml.silence@gmail.com


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