CVE-2026-23083 fou: Don't allow 0 for FOU_ATTR_IPPROTO.

In the Linux kernel, the following vulnerability has been resolved: fou: Don't allow 0 for FOUATTRIPPROTO. fouudprecv has the same problem mentioned in the previous patch. If FOUATTRIPPROTO is set to 0, skb is not freed by fouudprecv nor "resubmit"-ted in ipprotocoldeliverrcu. Let's forbid 0 for...