Refactor the key layer calculation function, in particular the tunnel key layer calculation by introducing helper functions. This is done in preparation for supporting GRE tunnel offloads. Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com> Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com> Reviewed-by: John Hurley <john.hurley@netronome.com> Signed-off-by: David S. Miller <davem@davemloft.net>