dccp: Prepare dccp_v4_route_skb() to .flowi4_tos conversion.

Use inet_sk_dscp() to get the socket DSCP value as dscp_t, instead of
ip_sock_rt_tos() which returns a __u8. This will ease the conversion
of fl4->flowi4_tos to dscp_t, which now just becomes a matter of
dropping the inet_dscp_to_dsfield() call.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/208dc5ca28bb5595d7a545de026bba18b1d63bda.1737032802.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Guillaume Nault

net/dccp/ipv4.c

@@ -15,6 +15,7 @@
 
 #include <net/icmp.h>
 #include <net/inet_common.h>
+#include <net/inet_dscp.h>
 #include <net/inet_hashtables.h>
 #include <net/inet_sock.h>
 #include <net/protocol.h>
@@ -473,7 +474,7 @@ static struct dst_entry* dccp_v4_route_skb(struct net *net, struct sock *sk,
 		.flowi4_oif = inet_iif(skb),
 		.daddr = iph->saddr,
 		.saddr = iph->daddr,
-		.flowi4_tos = ip_sock_rt_tos(sk),
+		.flowi4_tos = inet_dscp_to_dsfield(inet_sk_dscp(inet_sk(sk))),
 		.flowi4_scope = ip_sock_rt_scope(sk),
 		.flowi4_proto = sk->sk_protocol,
 		.fl4_sport = dccp_hdr(skb)->dccph_dport,