ipv4: Unmask upper DSCP bits when using hints

Unmask the upper DSCP bits when performing source validation and routing
a packet using the same route from a previously processed packet (hint).
In the future, this will allow us to perform the FIB lookup that is
performed as part of source validation according to the full DSCP value.

No functional changes intended since the upper DSCP bits are masked when
comparing against the TOS selectors in FIB rules and routes.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240821125251.1571445-13-idosch@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/route.c

@@ -2160,7 +2160,7 @@ int ip_route_use_hint(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	if (rt->rt_type != RTN_LOCAL)
 		goto skip_validate_source;
 
-	tos &= IPTOS_RT_MASK;
+	tos &= INET_DSCP_MASK;
 	err = fib_validate_source(skb, saddr, daddr, tos, 0, dev, in_dev, &tag);
 	if (err < 0)
 		goto martian_source;