rtnl_route: enhance RTA_TABLE decoding

The attribute of RTA_TABLE is not just a 32-bit integer, some constants
have well-known symbolic names like RT_TABLE_DEFAULT.

* nlattr.h (decode_nla_rt_class): New prototype.
* rtnl_route.c (decode_nla_rt_class): New function.
(rtmsg_nla_decoders) <RTA_TABLE>: Use it.
* tests/nlattr_rtmsg.c (main): Check RTA_TABLE decoding.

nlattr.h

@@ -62,5 +62,6 @@ DECL_NLA(str);
 DECL_NLA(strn);
 DECL_NLA(ifindex);
 DECL_NLA(meminfo);
+DECL_NLA(rt_class);
 
 #endif /* !STRACE_NLATTR_H */

rtnl_route.c

@@ -46,6 +46,21 @@
 #include "xlat/rtnl_route_attrs.h"
 #include "xlat/rtnl_rta_metrics_attrs.h"
 
+bool
+decode_nla_rt_class(struct tcb *const tcp,
+		    const kernel_ulong_t addr,
+		    const unsigned int len,
+		    const void *const opaque_data)
+{
+	uint32_t num;
+
+	if (len < sizeof(num))
+		return false;
+	if (!umove_or_printaddr(tcp, addr, &num))
+		printxval(routing_table_ids, num, NULL);
+	return true;
+}
+
 static bool
 decode_route_addr(struct tcb *const tcp,
 		  const kernel_ulong_t addr,
@@ -207,7 +222,7 @@ static const nla_decoder_t rtmsg_nla_decoders[] = {
 	[RTA_CACHEINFO]		= decode_rta_cacheinfo,
 	[RTA_SESSION]		= NULL, /* unused */
 	[RTA_MP_ALGO]		= decode_nla_u32,
-	[RTA_TABLE]		= decode_nla_u32,
+	[RTA_TABLE]		= decode_nla_rt_class,
 	[RTA_MARK]		= decode_nla_u32,
 	[RTA_MFC_STATS]		= decode_rta_mfc_stats,
 	[RTA_VIA]		= decode_rtvia,

tests/nlattr_rtmsg.c

@@ -106,6 +106,12 @@ main(void)
 			   RTA_OIF, pattern, ifindex,
 			   printf(IFINDEX_LO_STR));
 
+	const uint32_t rt_class_id = RT_TABLE_DEFAULT;
+	TEST_NLATTR_OBJECT(fd, nlh0, hdrlen,
+			   init_rtmsg, print_rtmsg,
+			   RTA_TABLE, pattern, rt_class_id,
+			   printf("RT_TABLE_DEFAULT"));
+
 	struct nlattr nla = {
 		.nla_type = RTAX_LOCK,
 		.nla_len = sizeof(nla)