net/colo.c: fix segmentation fault when packet is not parsed correctly

net/colo.c: fix segmentation fault when packet is not parsed correctly

When COLO use only one vnet_hdr_support parameter between
filter-redirector and filter-mirror(or colo-compare), COLO will crash
with segmentation fault. Back track as follow:

Thread 1 "qemu-system-x86" received signal SIGSEGV, Segmentation fault.
0x0000555555cb200b in eth_get_l2_hdr_length (p=0x0)
    at /home/tao/project/COLO/colo-qemu/include/net/eth.h:296
296         uint16_t proto = be16_to_cpu(PKT_GET_ETH_HDR(p)->h_proto);
(gdb) bt
0  0x0000555555cb200b in eth_get_l2_hdr_length (p=0x0)
    at /home/tao/project/COLO/colo-qemu/include/net/eth.h:296
1  0x0000555555cb22b4 in parse_packet_early (pkt=0x555556a44840) at
net/colo.c:49
2  0x0000555555cb2b91 in is_tcp_packet (pkt=0x555556a44840) at
net/filter-rewriter.c:63

So wrong vnet_hdr_len will cause pkt->data become NULL. Add check to
raise error and add trace-events to track vnet_hdr_len.

Signed-off-by: Tao Xu <tao3.xu@intel.com>
Signed-off-by: Zhang Chen <chen.zhang@intel.com>
Reviewed-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>