io_uring: remove unconditional looping in local task_work handling

If we have a ton of notifications coming in, we can be looping in here
for a long time. This can be problematic for various reasons, mostly
because we can starve userspace. If the application is waiting on N
events, then only re-run if we need more events.

Fixes: c0e0d6ba

 ("io_uring: add IORING_SETUP_DEFER_TASKRUN")
Signed-off-by: Jens Axboe <axboe@kernel.dk>