kernel: erspan: do not assume transport header is always set

In the Linux kernel, the following vulnerability has been resolved: erspan: do not assume transport header is always set Rewrite tests in ip6erspantunnelxmit and erspanfbxmit to not assume transport header is set. syzbot reported: WARNING: CPU: 0 PID: 1350 at include/linux/skbuff.h:2911...