eth/downloader: correct sync mode logging to show old mode (#30219)

This PR fixes an issue in the setMode method of beaconBackfiller where the
log message was not displaying the previous mode correctly. The log message
now shows both the old and new sync modes.

eth/downloader/beaconsync.go

@@ -123,7 +123,8 @@ func (b *beaconBackfiller) resume() {
 func (b *beaconBackfiller) setMode(mode SyncMode) {
 	// Update the old sync mode and track if it was changed
 	b.lock.Lock()
-	updated := b.syncMode != mode
+	oldMode := b.syncMode
+	updated := oldMode != mode
 	filling := b.filling
 	b.syncMode = mode
 	b.lock.Unlock()
@@ -133,7 +134,7 @@ func (b *beaconBackfiller) setMode(mode SyncMode) {
 	if !updated || !filling {
 		return
 	}
-	log.Error("Downloader sync mode changed mid-run", "old", mode.String(), "new", mode.String())
+	log.Error("Downloader sync mode changed mid-run", "old", oldMode.String(), "new", mode.String())
 	b.suspend()	
 	b.resume()
 }