io_uring: introduce io_is_compat()

A preparation patch adding a simple helper for gauging the compat state.
It'll help us to optimise and compile out more code in the following
commits.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://lore.kernel.org/r/1a87a640265196a67bc38300128e0bfd7839ab1f.1740400452.git.asml.silence@gmail.com


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