datapath: geneve: Refine MTU limit.

datapath: geneve: Refine MTU limit.

Upstream commit:
    Calculate the maximum MTU taking into account the size of headers
    involved in GENEVE encapsulation, as for other tunnel types.

    Changes in v3:
    - Correct comment style
    Changes in v2:
    - Conform more closely to ip_tunnel_change_mtu
    - Exclude GENEVE options from max MTU calculation

    Signed-off-by: David Wragg <david@weave.works>
    Acked-by: Jesse Gross <jesse@kernel.org>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Upstream: aeee0e66c6b4 ("geneve: Refine MTU limit")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>