io-wq: use kfree_rcu() to simplify the code

The callback function of call_rcu() just calls kfree(), so we can use
kfree_rcu() instead of call_rcu() + callback function.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>