datapath: vxlan: Relax MTU constraints.

datapath: vxlan: Relax MTU constraints.

Upstream commit:
    Allow the MTU of vxlan devices without an underlying device to be set
    to larger values (up to a maximum based on IP packet limits and vxlan
    overhead).

    Previously, their MTUs could not be set to higher than the
    conventional ethernet value of 1500.  This is a very arbitrary value
    in the context of vxlan, and prevented vxlan devices from being able
    to take advantage of jumbo frames etc.

    The default MTU remains 1500, for compatibility.

    Signed-off-by: David Wragg <david@weave.works>
    Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
Upstream: 72564b59ffc4 ("vxlan: Relax MTU constraints")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Jesse Gross <jesse@kernel.org>