io_uring: fix 'sync' handling of io_fallback_tw()

A previous commit added a 'sync' parameter to io_fallback_tw(), which if
true, means the caller wants to wait on the fallback thread handling it.
But the logic is somewhat messed up, ensure that ctxs are swapped and
flushed appropriately.

Cc: stable@vger.kernel.org
Fixes: dfbe5561 ("io_uring: flush offloaded and delayed task_work on exit")
Signed-off-by: Jens Axboe <axboe@kernel.dk>