xfrm: fix bug with DSCP copy to v6 from v4 tunnel

When copying the DSCP bits for decap-dscp into IPv6 don't assume the
outer encap is always IPv6. Instead, as with the inner IPv4 case, copy
the DSCP bits from the correctly saved "tos" value in the control block.

Fixes: 227620e2 ("[IPSEC]: Separate inner/outer mode processing on input")
Signed-off-by: Christian Hopps <chopps@chopps.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>