net: geneve: set IFF_POINTOPOINT with IFLA_GENEVE_INNER_PROTO_INHERIT

The GENEVE tunnel used with IFLA_GENEVE_INNER_PROTO_INHERIT is
point-to-point, so set IFF_POINTOPOINT to reflect that.

Signed-off-by: Josef Miegl <josef@miegl.cz>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>