ip6_gre: update version related info when changing link

We forgot to update ip6erspan version related info when changing link,
which will cause setting new hwid failed.

Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: 94d7d8f2

 ("ip6_gre: add erspan v2 support")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>