gtp: Prepare ip4_route_output_gtp() 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: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/06bdb310a075355ff059cd32da2efc29a63981c9.1737034675.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Guillaume Nault

drivers/net/gtp.c

@@ -23,6 +23,8 @@
 
 #include <net/net_namespace.h>
 #include <net/protocol.h>
+#include <net/inet_dscp.h>
+#include <net/inet_sock.h>
 #include <net/ip.h>
 #include <net/ipv6.h>
 #include <net/udp.h>
@@ -350,7 +352,7 @@ static struct rtable *ip4_route_output_gtp(struct flowi4 *fl4,
 	fl4->flowi4_oif		= sk->sk_bound_dev_if;
 	fl4->daddr		= daddr;
 	fl4->saddr		= saddr;
-	fl4->flowi4_tos		= ip_sock_rt_tos(sk);
+	fl4->flowi4_tos		= inet_dscp_to_dsfield(inet_sk_dscp(inet_sk(sk)));
 	fl4->flowi4_scope	= ip_sock_rt_scope(sk);
 	fl4->flowi4_proto	= sk->sk_protocol;