io_uring: introduce io_has_work

This will be used later to know if the ring has outstanding work. Right
now just if there is overflow CQEs to copy to the main CQE ring, but later
will include deferred tasks

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220830125013.570060-3-dylany@fb.com


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