Extract from ip6gre_changelink() a reusable function
ip6gre_changelink_common(). This will allow introduction of
ERSPAN-specific _changelink() function with not a lot of code
duplication.
Fixes: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support") Signed-off-by: Petr Machata <petrm@mellanox.com> Acked-by: William Tu <u9012063@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>