iomap: allocate s_dio_done_wq for async reads as well

In the Linux kernel, the following vulnerability has been resolved: iomap: allocate s_dio_done_wq for async reads as well Since commit 222f2c7c6d14 ("iomap: always run error completions in user context"), read error completions are deferred to s_dio_done_wq. This means the workqueue also needs to be allocated for async reads.