vxlan: allow L4 GRO passthrough

When passing up an UDP GSO packet with L4 aggregation, there is
no need to segment it at the vxlan level. We can propagate the
packet untouched and let it be segmented later, if needed.

Introduce an helper to allow let the UDP socket to accept any
L4 aggregation and use it in the vxlan driver.

v1 -> v2:
 - updated to use the newly introduced UDP socket 'accept*' fields

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>