nfp: flower: remove unused tun_mask variable

The tunnel dest IP is required for separate offload to the NFP. It is
already verified that a dest IP must be present and must be an exact
match in the flower rule. Therefore, we can just extract the IP from the
generated offload rule and remove the unused mask variable. The function
is then no longer required to return the IP separately.

Because tun_dst is localised to tunnel matches, move the declaration to
the tunnel if branch.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>