io_uring: ensure overflow entries are dropped when ring is exiting

io_uring: ensure overflow entries are dropped when ring is exiting

A previous consolidation cleanup missed handling the case where the ring
is dying, and __io_cqring_overflow_flush() doesn't flush entries if the
CQ ring is already full. This is fine for the normal CQE overflow
flushing, but if the ring is going away, we need to flush everything,
even if it means simply freeing the overflown entries.

Fixes: 6c948ec44b29 ("io_uring: consolidate overflow flushing")
Signed-off-by: Jens Axboe <axboe@kernel.dk>