Open vSwitch provides code to pop an MPLS header to a packet. In
preparation for supporting this in TC, move the pop code to an skb helper
that can be reused.

Remove the, now unused, update_ethertype static function from OvS.
Signed-off-by: NJohn Hurley <john.hurley@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NSimon Horman <simon.horman@netronome.com>
Reviewed-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>