Linux: devlink: fix devlink_rel reference leak when notify work is pending

In the Linux kernel, the following vulnerability has been resolved:

devlink: fix devlink_rel reference leak when notify work is pending

devlink_rel_nested_in_notify_work_schedule() takes a reference on the devlink_rel for the notify work and then queues the work, ignoring the return value of schedule_delayed_work(). If the work is already pending, nothing new is queued, the work runs only once and drops only one reference, so the extra one is leaked together with the devlink_rel and its index in devlink_rels.

This is easy to hit. devl_register() of a nested instance queues the work, and if the instance is unregistered before the work has run, devlink_rel_put() queues it again while it is still pending. The work also keeps rescheduling itself for as long as the parent devlink lock cannot be taken, which widens the window. Registering and unregistering a nested devlink instance 100 times while holding the parent lock leaks all 100 devlink_rel objects.

The reschedule path in devlink_rel_nested_in_notify_work() has the same problem: if the work was queued again while it was running, the reference it holds is never dropped.

Drop the reference in both places when the work was already pending. The pending work holds its own reference, so this can not be the last one.