bpf: fix bpf_skb_set_tunnel_key() helper

Make sure to indicate to tunnel driver that key.tun_id is set,
otherwise gre won't recognize the metadata.

Fixes: d3aa45ce ("bpf: add helpers to access tunnel metadata")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>