net-ipv6: changes to ->tclass (via IPV6_TCLASS) should sk_dst_reset()

This is to match ipv4 behaviour, see __ip_sock_set_tos()
implementation.

Technically for ipv6 this might not be required because normally we
do not allow tclass to influence routing, yet the cli tooling does
support it:

lpk11:~# ip -6 rule add pref 5 tos 45 lookup 5
lpk11:~# ip -6 rule
5:      from all tos 0x45 lookup 5

and in general dscp/tclass based routing does make sense.

We already have cases where dscp can affect vlan priority and/or
transmit queue (especially on wifi).

So let's just make things match.  Easier to reason about and no harm.

Cc: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Link: https://lore.kernel.org/r/20211123223208.1117871-1-zenczykowski@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ipv6_sockglue.c

@@ -603,7 +603,10 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 			val &= ~INET_ECN_MASK;
 			val |= np->tclass & INET_ECN_MASK;
 		}
-		np->tclass = val;
+		if (np->tclass != val) {
+			np->tclass = val;
+			sk_dst_reset(sk);
+		}
 		retv = 0;
 		break;