sctp: Prepare sctp_v4_get_dst() to dscp_t conversion.

Define inet_sk_dscp() to get a dscp_t value from struct inet_sock, so
that sctp_v4_get_dst() can easily set ->flowi4_tos from a dscp_t
variable. For the SCTP_DSCP_SET_MASK case, we can just use
inet_dsfield_to_dscp() to get a dscp_t value.

Then, when converting ->flowi4_tos from __u8 to dscp_t, we'll just have
to drop the inet_dscp_to_dsfield() conversion function.

Signed-off-by: Guillaume Nault <[e-mail]>
Acked-by: Xin Long <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/inet_sock.h

···
 #include <linux/netdevice.h>
 
 #include <net/flow.h>
+#include <net/inet_dscp.h>
 #include <net/sock.h>
 #include <net/request_sock.h>
 #include <net/netns/hash.h>
···
 static inline unsigned long inet_cmsg_flags(const struct inet_sock *inet)
 {
 	return READ_ONCE(inet->inet_flags) & IP_CMSG_ALL;
+}
+
+static inline dscp_t inet_sk_dscp(const struct inet_sock *inet)
+{
+	return inet_dsfield_to_dscp(READ_ONCE(inet->tos));
 }
 
 #define inet_test_bit(nr, sk)			\

net/sctp/protocol.c

···
 #include <net/addrconf.h>
 #include <net/inet_common.h>
 #include <net/inet_ecn.h>
+#include <net/inet_sock.h>
 #include <net/udp_tunnel.h>
 #include <net/inet_dscp.h>
 
···
 	struct dst_entry *dst = NULL;
 	union sctp_addr *daddr = &t->ipaddr;
 	union sctp_addr dst_saddr;
-	u8 tos = READ_ONCE(inet_sk(sk)->tos);
+	dscp_t dscp;
 
 	if (t->dscp & SCTP_DSCP_SET_MASK)
-		tos = t->dscp & SCTP_DSCP_VAL_MASK;
+		dscp = inet_dsfield_to_dscp(t->dscp);
+	else
+		dscp = inet_sk_dscp(inet_sk(sk));
+
 	memset(&_fl, 0x0, sizeof(_fl));
 	fl4->daddr  = daddr->v4.sin_addr.s_addr;
 	fl4->fl4_dport = daddr->v4.sin_port;
 	fl4->flowi4_proto = IPPROTO_SCTP;
 	if (asoc) {
-		fl4->flowi4_tos = tos & INET_DSCP_MASK;
+		fl4->flowi4_tos = inet_dscp_to_dsfield(dscp);
 		fl4->flowi4_scope = ip_sock_rt_scope(asoc->base.sk);
 		fl4->flowi4_oif = asoc->base.sk->sk_bound_dev_if;
 		fl4->fl4_sport = htons(asoc->base.bind_addr.port);