ipv4: ipmr: Unmask upper DSCP bits in ipmr_rt_fib_lookup()

Unmask the upper DSCP bits when calling ipmr_fib_lookup() so that in the
future it could perform the FIB lookup according to the full DSCP value.

Note that ipmr_fib_lookup() performs a FIB rule lookup (returning the
relevant routing table) and that IPv4 multicast FIB rules do not support
matching on TOS / DSCP. However, it is still worth unmasking the upper
DSCP bits in case support for DSCP matching is ever added.

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/20240821125251.1571445-7-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ipmr.c

@@ -62,6 +62,7 @@
 #include <net/fib_rules.h>
 #include <linux/netconf.h>
 #include <net/rtnh.h>
+#include <net/inet_dscp.h>
 
 #include <linux/nospec.h>
 
@@ -2080,7 +2081,7 @@ static struct mr_table *ipmr_rt_fib_lookup(struct net *net, struct sk_buff *skb)
 	struct flowi4 fl4 = {
 		.daddr = iph->daddr,
 		.saddr = iph->saddr,
-		.flowi4_tos = RT_TOS(iph->tos),
+		.flowi4_tos = iph->tos & INET_DSCP_MASK,
 		.flowi4_oif = (rt_is_output_route(rt) ?
 			       skb->dev->ifindex : 0),
 		.flowi4_iif = (rt_is_output_route(rt) ?