xfrm6: Fix IPv6 ECN decapsulation

xfrm6: Fix IPv6 ECN decapsulation

Using ipv6_get_dsfield on the outer IP header implies that inner and
outer header are of the the same address family. For interfamily
tunnels, particularly 646, the code reading the DSCP field obtains the
wrong values (IHL and the upper four bits of the DSCP field).
This can cause the code to detect a congestion encoutered state in the
outer header and enable the corresponding bits in the inner header, too.

Since the DSCP field is stored in the xfrm mode common buffer
independently from the IP version of the outer header, it's safe (and
correct) to take this value from there.

Signed-off-by: Thomas Egerer <thomas.egerer@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>