batman-adv: bla: put backbone reference on failed claim hash insert

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

batman-adv: bla: put backbone reference on failed claim hash insert

When batadv_bla_add_claim() fails to insert a new claim into the hash, it

leaked a reference to the backbone_gw for which the claim was intended.

Call batadv_backbone_gw_put() on the error path to release the reference

and avoid leaking the backbone_gw object.