io_uring: dedup io_run_task_work

We have an identical copy of io_run_task_work() for io-wq called
io_flush_signals(), deduplicate them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a157a4df5fa217b8bd03c73494f2fd0e24e44fbc.1655802465.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>