tunnels: Allow IPv6 UDP checksums to be correctly controlled.

When configuring checksums on UDP tunnels, the flags are different
for IPv4 vs. IPv6 (and reversed). However, when lightweight tunnels
are enabled the flags used are always the IPv4 versions, which are
ignored in the IPv6 code paths. This uses the correct IPv6 flags, so
checksums can be controlled appropriately.

Fixes: a725e514 ("vxlan: metadata based tunneling for IPv6")
Fixes: abe492b4 ("geneve: UDP checksum configuration via netlink")
Signed-off-by: Jesse Gross <jesse@kernel.org>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>