datapath: Set a large MTU on tunnel devices.

datapath: Set a large MTU on tunnel devices.

Upstream commit:
    Prior to 4.3, openvswitch tunnel vports (vxlan, gre and geneve) could
    transmit vxlan packets of any size, constrained only by the ability to
    send out the resulting packets.  4.3 introduced netdevs corresponding
    to tunnel vports.  These netdevs have an MTU, which limits the size of
    a packet that can be successfully encapsulated.  The default MTU
    values are low (1500 or less), which is awkwardly small in the context
    of physical networks supporting jumbo frames, and leads to a
    conspicuous change in behaviour for userspace.

    Instead, set the MTU on openvswitch-created netdevs to be the relevant
    maximum (i.e. the maximum IP packet size minus any relevant overhead),
    effectively restoring the behaviour prior to 4.3.

    Signed-off-by: David Wragg <david@weave.works>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Upstream: 7e059158d57b ("vxlan, gre, geneve: Set a large MTU on ovs-created
tunnel devices")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>