gre: Prepare ipgre_open() to .flowi4_tos conversion.

Use ip4h_dscp() to get the tunnel DSCP option as dscp_t, instead of
manually masking the raw tos field with INET_DSCP_MASK. This will ease
the conversion of fl4->flowi4_tos to dscp_t, which just becomes a
matter of dropping the inet_dscp_to_dsfield() call.

Signed-off-by: Guillaume Nault <[email protected]>
Reviewed-by: Ido Schimmel <[email protected]>
Link: https://patch.msgid.link/6c05a11afdc61530f1a4505147e0909ad51feb15.1736941806.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Guillaume Nault

net/ipv4/ip_gre.c

@@ -926,7 +926,7 @@ static int ipgre_open(struct net_device *dev)
 	if (ipv4_is_multicast(t->parms.iph.daddr)) {
 		struct flowi4 fl4 = {
 			.flowi4_oif = t->parms.link,
-			.flowi4_tos = t->parms.iph.tos & INET_DSCP_MASK,
+			.flowi4_tos = inet_dscp_to_dsfield(ip4h_dscp(&t->parms.iph)),
 			.flowi4_scope = RT_SCOPE_UNIVERSE,
 			.flowi4_proto = IPPROTO_GRE,
 			.saddr = t->parms.iph.saddr,