geneve: fix populating tclass in geneve_get_v6_dst

The struct flowi6's flowi6_tos is not used in IPv6 route lookup, the
traffic class information is handled in the flowi6's flowlabel member
instead. For example, for policy routing, fib6_rule_match() uses
ip6_tclass() that is applied on the flowlabel for matching on tclass,
which would currently not work as expected.

Fixes: 3a56f86f ("geneve: handle ipv6 priority like ipv4 tos")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>