devlink: Release nested relation on devlink free

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

devlink: Release nested relation on devlink free

devlink relation state is normally released from devl_unregister(), which

calls devlink_rel_put(). This misses devlink instances that get a nested

relation before registration and then fail probe before devl_register() is

reached.

That flow can happen for SFs. The child devlink gets linked to its

parent before registration, then a later probe error calls devlink_free()

directly. Since the instance was never registered, devl_unregister() is not

called and devlink->rel is leaked.

Release any pending relation from devlink_free() as well. The registered

path is unchanged because devl_unregister() already clears devlink->rel

before devlink_free() runs.