For BPF_PROG_TYPE_SCHED_CLS/ACT, the upstream kernel has recently added a
feature to efficiently output skb + meta data:
commit 555c8a8623a3 ("bpf: avoid stack copy and use skb ctx for event output")

This patch adds perf_submit_skb to BPF_PERF_OUTPUT macro.  It takes
an extra u32 argument.  perf_submit_skb will then be expanded to
bpf_perf_event_output properly to consider the newly added
u32 argument as the skb's len.

Other than the above described changes, perf_submit_skb is almost
a carbon copy of the perf_submit except the removal of the 'string name'
variable since I cannot find a specific use of it.

Note that the 3rd param type of bpf_perf_event_output has also been
changed from u32 to u64.

Added a sample program tc_perf_event.py.  Here is how the output
looks like:
[root@arch-fb-vm1 networking]# ./tc_perf_event.py
Try: "ping -6 ff02::1%me"

CPU SRC IP                           DST IP       Magic
0   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c
0   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c
0   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c
1   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c
1   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c
1   fe80::982f:5dff:fec1:e52b        ff02::1      0xfaceb00c