md/raid10: wait barrier before returning discard request with REQ_NOWAIT

In the Linux kernel, the following vulnerability has been resolved: md/raid10: wait barrier before returning discard request with REQ_NOWAIT raid10_handle_discard should wait barrier before returning a discard bio which has REQ_NOWAIT. And there is no need to print warning calltrace if a discard bio has REQ_NOWAIT flag. Quality engineer usually checks dmesg and reports error if dmesg has warning/error calltrace.