repart: Flush varlink messages for progress updates

Progress updates are not send out immidialty and only once the handler
for the run method completes since the event loop is not processed while
the run handler is executed. Therefore flush varlink messages immidialty
after a progress update is queued.

Closes: github.com/systemd/systemd/issues/40238

src/repart/repart.c

@@ -2905,6 +2905,10 @@ static int context_notify(
                                 JSON_BUILD_PAIR_UNSIGNED_NOT_EQUAL("progress", percent, UINT_MAX));
                 if (r < 0)
                         log_debug_errno(r, "Failed to send varlink notify progress notification, ignoring: %m");
+
+                r = sd_varlink_flush(c->link);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to flush varlink notify progress notification, ignoring: %m");
         }
 
         return 0;