bpf: add skb->tstamp r/w access from tc clsact and cg skb progs

This could be used to rate limit egress traffic in concert with a qdisc
which supports Earliest Departure Time, such as FQ.

Write access from cg skb progs only with CAP_SYS_ADMIN, since the value
will be used by downstream qdiscs. It might make sense to relax this.

Changes v1 -> v2:
  - allow access from cg skb, write only with CAP_SYS_ADMIN

Signed-off-by: Vlad Dumitrescu <vladum@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>