gre: fix ToS/DiffServ inherit bug

Fixes two bugs:
- ToS/DiffServ inheritance was unintentionally activated when using impair fixed ToS values
- ECN bit was lost during ToS/DiffServ inheritance

Signed-off-by: Andreas Jaggi <aj@open.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>