bpf: add bpf_skb_copy_bytes() helper Since ingress and egress programs see different skb->data, add bpf_skb_copy_bytes() helper which is skb->mac_header relative as a substitute for LD_ABS/LD_IND instructions which are skb->data relative Fix all other helpers to be skb->mac_header relative Ensure that mac_header is set before calling into programs Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>