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

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.

Fixes: f975a0955276 ("net: mana: Fix double destroy_workqueue on service rescan PCI path")
Signed-off-by: Shiraz Saleem <shirazsaleem@microsoft.com>
Signed-off-by: Konstantin Taranov <kotaranov@microsoft.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260309172443.688392-1-kotaranov@linux.microsoft.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>