rtnl_rule: enhance FRA_TABLE decoding

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

* rtnl_rule.c (fib_rule_hdr_nla_decoders) <FRA_TABLE>:
Use decode_nla_rt_class.
* tests/nlattr_fib_rule_hdr.c (FRA_TABLE): New macro.
(main): Check FRA_TABLE decoding.

rtnl_rule.c

@@ -90,7 +90,7 @@ static const nla_decoder_t fib_rule_hdr_nla_decoders[] = {
 	[FRA_TUN_ID]			= decode_nla_be64,
 	[FRA_SUPPRESS_IFGROUP]		= decode_nla_u32,
 	[FRA_SUPPRESS_PREFIXLEN]	= decode_nla_u32,
-	[FRA_TABLE]			= decode_nla_u32,
+	[FRA_TABLE]			= decode_nla_rt_class,
 	[FRA_FWMASK]			= decode_nla_u32,
 	[FRA_OIFNAME]			= decode_nla_str,
 	[FRA_PAD]			= NULL,

tests/nlattr_fib_rule_hdr.c

@@ -38,6 +38,7 @@
 # include <linux/rtnetlink.h>
 
 #define FRA_TUN_ID 12
+#define FRA_TABLE 15
 #define FRA_UID_RANGE 20
 
 static void
@@ -98,6 +99,12 @@ main(void)
 		    FRA_DST, 4, pattern, 4,
 		    print_quoted_hex(pattern, 4));
 
+	const uint32_t table_id = RT_TABLE_DEFAULT;
+	TEST_NLATTR_OBJECT(fd, nlh0, hdrlen,
+			   init_rtmsg, print_rtmsg,
+			   FRA_TABLE, pattern, table_id,
+			   printf("RT_TABLE_DEFAULT"));
+
 #ifdef HAVE_STRUCT_FIB_RULE_UID_RANGE
 	static const struct fib_rule_uid_range range = {
 		.start = 0xabcdedad,