ip/ip6tunnel: fix update of tclass and flowlabel

When tclass or flowlabel field were updated, we only performed an OR with the
new value. For example, it was not possible to reset tclass:
  ip -6 tunnel change ip6tnl2 tclass 0
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>

ip/ip6tunnel.c

@@ -173,6 +173,7 @@ static int parse_args(int argc, char **argv, int cmd, struct ip6_tnl_parm *p)
 			   matches(*argv, "dsfield") == 0) {
 			__u8 uval;
 			NEXT_ARG();
+			p->flowinfo &= ~IP6_FLOWINFO_TCLASS;
 			if (strcmp(*argv, "inherit") == 0)
 				p->flags |= IP6_TNL_F_USE_ORIG_TCLASS;
 			else {
@@ -185,6 +186,7 @@ static int parse_args(int argc, char **argv, int cmd, struct ip6_tnl_parm *p)
 			   strcmp(*argv, "fl") == 0) {
 			__u32 uval;
 			NEXT_ARG();
+			p->flowinfo &= ~IP6_FLOWINFO_FLOWLABEL;
 			if (strcmp(*argv, "inherit") == 0)
 				p->flags |= IP6_TNL_F_USE_ORIG_FLOWLABEL;
 			else {