ip6_tunnel: Correct tos value in collect_md mode

Same as ip_gre, geneve and vxlan, use key->tos as traffic class value.

CC: Peter Dawson <petedaws@gmail.com>
Fixes: 0e9a7095 ("ip6_tunnel, ip6_gre: fix setting of DSCP on
encapsulated packets”)
Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Acked-by: Peter Dawson <peter.a.dawson@boeing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>