CVE-2026-72228: batman-adv: frag: fix primary_if leak on failed linearization

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

batman-adv: frag: fix primaryif leak on failed linearization

If the skb has a fraglist, it must be linearized before it can be split using skbsplit(). But when this step failed, it must not only free the skb but also take care of the reference to the already found primaryif.