io_uring: use constants for cq_overflow bitfield

Prepare to use this bitfield for more flags by using constants instead of
magic value 0

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220421091345.2115755-5-dylany@fb.com


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