batman-adv: bla: only purge non-released claims

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

batman-adv: bla: only purge non-released claims

When batadv_bla_purge_claims() goes through the list of claims, it is only

traversing the hash list with an rcu_read_lock(). Due to a potential

parallel batadv_claim_put(), it can happen that it encounters a claim which

was actually in the process of being released+freed by

batadv_claim_release(). In this case, backbone_gw is set to NULL before the

delayed RCU kfree is started. Calling batadv_bla_claim_get_backbone_gw() is

then no longer allowed because it would cause a NULL-ptr derefence.

To avoid this, only claims with a valid reference counter must be purged.

All others are already taken care of.