bpf: fix calculation od vxlan udp size
bpf: fix calculation od vxlan udp size

we cannot trust the tail pointer, must be calculated from something
reliable like the ip->tot_len - the size of the ip header.

neiljerram/felix,neiljerram/felix,neiljerram/felix,Metaswitch/calico,Metaswitch/calico,neiljerram/felix