Add ActivityGuard::update() for in-place progress updates

Lets long-running operations update their status bar message without
creating/dropping a new activity per iteration. Useful for loops
like memory scoring where you want "scoring: 3/25 keyname" updating
in place.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/mod.rs

@@ -42,6 +42,17 @@ pub struct ActivityGuard {
     id: u64,
 }
 
+impl ActivityGuard {
+    pub async fn update(&self, label: impl Into<String>) {
+        let label = label.into();
+        let mut st = self.agent.state.lock().await;
+        if let Some(entry) = st.activities.iter_mut().find(|a| a.id == self.id) {
+            entry.label = label;
+        }
+        st.changed.notify_one();
+    }
+}
+
 const ACTIVITY_LINGER: std::time::Duration = std::time::Duration::from_secs(5);
 
 impl Drop for ActivityGuard {