CVE-2022-49663 tunnels: do not assume mac header is set in skb_tunnel_check_pmtu()

In the Linux kernel, the following vulnerability has been resolved: tunnels: do not assume mac header is set in skbtunnelcheckpmtu Recently added debug in commit f9aefd6b2aa3 "net: warn if mac header was not set" caught a bug in skbtunnelcheckpmtu, as shown in this syzbot report 1. In ndostartxmi...