IPv4 UDP and GRE tunnel match rule compile helpers share functions for
compiling fields such as IP addresses. However, they handle fields such
tunnel IDs differently.

Create new helper functions for compiling GRE and UDP tunnel key data.
This is in preparation for supporting IPv6 tunnels where these new
functions can be reused.

This patch does not change functionality.
Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>