geneve: Relax MTU constraints

Allow the MTU of geneve devices to be set to large values, in order to
exploit underlying networks with larger frame sizes.

GENEVE does not have a fixed encapsulation overhead (an openvswitch
rule can add variable length options), so there is no relevant maximum
MTU to enforce.  A maximum of IP_MAX_MTU is used instead.
Encapsulated packets that are too big for the underlying network will
get dropped on the floor.

Signed-off-by: David Wragg <david@weave.works>
Signed-off-by: David S. Miller <davem@davemloft.net>