gre: add collect_md mode to ERSPAN tunnel

Similar to gre, vxlan, geneve, ipip tunnels, allow ERSPAN tunnels to
operate in 'collect metadata' mode.  bpf_skb_[gs]et_tunnel_key() helpers
can make use of it right away.  OVS can use it as well in the future.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>