virtio-devices: block: track non-batch inflight reqs immediately

For non-batch backends execute_async submits the kernel I/O inline
before returning. An early return while processing before inserting in
inflight_requests, meant the request went untracked, the local batch
list was never appended to inflight_requests, even though the request is
pending in the kernel.

To track it, insert into self.inflight_requests as soon as execute_async
returns Ok. The completion path's find_inflight_request now matches the
orphan and the bounce buffer is freed only after the kernel signals it
is done.

Signed-off-by: Dylan Reid <dgreid@fb.com>

virtio-devices/src/block.rs

@@ -326,8 +326,11 @@ impl BlockEpollHandler {
                             )
                         }
                     }
+                    batch_inflight_requests.push((desc_chain.head_index(), request));
+                } else {
+                    self.inflight_requests
+                        .push_back((desc_chain.head_index(), request));
                 }
-                batch_inflight_requests.push((desc_chain.head_index(), request));
             } else {
                 let status = match result {
                     Ok(_) => VIRTIO_BLK_S_OK,