system/qdev-monitor: move drain_call_rcu call under if (!dev) in qmp_device_add()

Original goal of addition of drain_call_rcu to qmp_device_add was to cover
the failure case of qdev_device_add. It seems call of drain_call_rcu was
misplaced in 7bed89958bfbf40df what led to waiting for pending RCU callbacks
under happy path too. What led to overall performance degradation of
qmp_device_add.

In this patch call of drain_call_rcu moved under handling of failure of
qdev_device_add.

Signed-off-by: Dmitrii Gavrilov <ds-gavr@yandex-team.ru>
Message-ID: <20231103105602.90475-1-ds-gavr@yandex-team.ru>
Fixes: 7bed89958bf ("device_core: use drain_call_rcu in in qmp_device_add", 2020-10-12)
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

system/qdev-monitor.c

@@ -858,19 +858,18 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
         return;
     }
     dev = qdev_device_add(opts, errp);
-
-    /*
-     * Drain all pending RCU callbacks. This is done because
-     * some bus related operations can delay a device removal
-     * (in this case this can happen if device is added and then
-     * removed due to a configuration error)
-     * to a RCU callback, but user might expect that this interface
-     * will finish its job completely once qmp command returns result
-     * to the user
-     */
-    drain_call_rcu();
-
     if (!dev) {
+        /*
+         * Drain all pending RCU callbacks. This is done because
+         * some bus related operations can delay a device removal
+         * (in this case this can happen if device is added and then
+         * removed due to a configuration error)
+         * to a RCU callback, but user might expect that this interface
+         * will finish its job completely once qmp command returns result
+         * to the user
+         */
+        drain_call_rcu();
+
         qemu_opts_del(opts);
         return;
     }