io_uring: check if we need to reschedule during overflow flush

In terms of normal application usage, this list will always be empty.
And if an application does overflow a bit, it'll have a few entries.
However, nothing obviously prevents syzbot from running a test case
that generates a ton of overflow entries, and then flushing them can
take quite a while.

Check for needing to reschedule while flushing, and drop our locks and
do so if necessary. There's no state to maintain here as overflows
always prune from head-of-list, hence it's fine to drop and reacquire
the locks at the end of the loop.

Link: https://lore.kernel.org/io-uring/66ed061d.050a0220.29194.0053.GAE@google.com/


Reported-by:  <syzbot+5fca234bd7eb378ff78e@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>