block_util: Avoid intermediate completion queue allocation

Rather than aggregate the completion list into an intermediate vector
instead adjust the API to provide one completion item at a time.

With DHAT this shows the number of heap allocations has decreased.

Before:

    dhat: Total:     623,852 bytes in 8,157 blocks

After:

    dhat: Total:     380,444 bytes in 3,469 blocks

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

block_util/src/raw_async.rs

@@ -155,13 +155,10 @@ impl AsyncIo for RawFileAsync {
         Ok(())
     }
 
-    fn complete(&mut self) -> Vec<(u64, i32)> {
-        let cq = self.io_uring.completion();
-        let mut completion_list = Vec::with_capacity(cq.len());
-        for cq_entry in cq {
-            completion_list.push((cq_entry.user_data(), cq_entry.result()));
-        }
-
-        completion_list
+    fn next_completed_request(&mut self) -> Option<(u64, i32)> {
+        self.io_uring
+            .completion()
+            .next()
+            .map(|entry| (entry.user_data(), entry.result()))
     }
 }