net/mana: Null service_wq on setup error to prevent double destroy

In the Linux kernel, the following vulnerability has been resolved: net/mana: Null service_wq on setup error to prevent double destroy In mana_gd_setup() error path, set gc->service_wq to NULL after destroy_workqueue() to match the cleanup in mana_gd_cleanup(). This prevents a use-after-free if the workqueue pointer is checked after a failed setup.