netfilter: IPv6: fix DSCP mangle code

commit 1ed2f73d upstream.

The mask indicates the bits one wants to zero out, so it needs to be
inverted before applying to the original TOS field.

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Andi Kleen <ak@linux.intel.com>