bpf: make bpf_skb_ecn_set_ce callable from BPF_PROG_TYPE_SCHED_ACT

This helper is useful if a bpf tc filter sets skb->tstamp.

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>