block: rnull: implement dummy timeout callback

Implement the new Operations::timeout callback for rnull and return
TimeoutReturn::ResetTimer.

Using ResetTimer keeps the behavior close to the existing blk-mq
default timeout handling while proving that the Rust timeout
abstraction wires cleanly into a driver.

Signed-off-by: Wenzhao Liao <[email protected]>

Author: lwz23

drivers/block/rnull/rnull.rs

@@ -97,4 +97,8 @@ impl Operations for NullBlkDevice {
             // point, and so `end_ok` cannot fail.
             .expect("Fatal error - expected to be able to end request");
     }
+
+    fn timeout(_rq: &mq::Request<Self>) -> mq::TimeoutReturn {
+        mq::TimeoutReturn::ResetTimer
+    }
 }