ip6_gre: ip6gre_tap device should keep dst

The patch 'ip_gre: ipgre_tap device should keep dst' fixed
a issue that ipgre_tap mtu couldn't be updated in tx path.

The same fix is needed for ip6gre_tap as well.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>