lwt: fix rx checksum setting for lwt devices tunneling over ipv6

the commit 35e2d115 ("tunnels: Allow IPv6 UDP checksums to be
correctly controlled.") changed the default xmit checksum setting
for lwt vxlan/geneve ipv6 tunnels, so that now the checksum is not
set into external UDP header.
This commit changes the rx checksum setting for both lwt vxlan/geneve
devices created by openvswitch accordingly, so that lwt over ipv6
tunnel pairs are again able to communicate with default values.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Acked-by: Jesse Gross <jesse@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>