io_uring/io-wq: remove io_wq_for_each_worker() return value

io_uring/io-wq: remove io_wq_for_each_worker() return value

The only use of this helper is to iterate all of the workers, and
hence all callers will pass in a func that always returns false to do
that. As none of the callers use the return value, get rid of it.

Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>