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.