netlink/route: extend pre-2.6.19 Linux compat shim to del/getroute

Commit f34aca55ad ("netlink/route: provide pre-2.6.19 Linux compat shim",
2024-06) fixed the partial fix for net/bird2 on the netlink path by mapping the
legacy 8-bit struct rtmsg::rtm_table field onto the modern 32-bit RTA_TABLE
attribute when the latter is absent.

That fix, however, was only applied to rtnl_handle_newroute. The two sibling
handlers: rtnl_handle_delroute and rtnl_handle_getroute were left looking at
attrs.rta_table directly. They are reachable from exactly the same client
(bird, in its netlink scan path), so any FIB number that fits in 8 bits
silently maps to RT_TABLE_UNSPEC in those handlers.

Reviewed by:	melifaro (previous version)
Approved by:	emaste
MFC after:	1 week
Sponsored by:	Netflix

sys/netlink/route/rt.c

@@ -978,6 +978,14 @@ create_nexthop_from_attrs(struct nl_parsed_route *attrs,
 	return (nh);
 }
 
+/* pre-2.6.19 Linux API compatibility: prefer RTA_TABLE, fall back to rtm_table */
+static inline void
+old_linux_compat(struct nl_parsed_route *attrs)
+{
+	if (attrs->rtm_table > 0 && attrs->rta_table == 0)
+		attrs->rta_table = attrs->rtm_table;
+}
+
 static int
 rtnl_handle_newroute(struct nlmsghdr *hdr, struct nlpcb *nlp,
     struct nl_pstate *npt)
@@ -997,9 +1005,7 @@ rtnl_handle_newroute(struct nlmsghdr *hdr, struct nlpcb *nlp,
 		return (EINVAL);
 	}
 
-	/* pre-2.6.19 Linux API compatibility */
+	old_linux_compat(&attrs);
-	if (attrs.rtm_table > 0 && attrs.rta_table == 0)
-		attrs.rta_table = attrs.rtm_table;
 	if (attrs.rta_table >= V_rt_numfibs || attrs.rtm_family > AF_MAX) {
 		NLMSG_REPORT_ERR_MSG(npt, "invalid fib");
 		return (EINVAL);
@@ -1066,6 +1072,7 @@ rtnl_handle_delroute(struct nlmsghdr *hdr, struct nlpcb *nlp,
 		return (ESRCH);
 	}
 
+	old_linux_compat(&attrs);
 	if (attrs.rta_table >= V_rt_numfibs || attrs.rtm_family > AF_MAX) {
 		NLMSG_REPORT_ERR_MSG(npt, "invalid fib");
 		return (EINVAL);
@@ -1089,6 +1096,7 @@ rtnl_handle_getroute(struct nlmsghdr *hdr, struct nlpcb *nlp, struct nl_pstate *
 	if (error != 0)
 		return (error);
 
+	old_linux_compat(&attrs);
 	if (attrs.rta_table >= V_rt_numfibs || attrs.rtm_family > AF_MAX) {
 		NLMSG_REPORT_ERR_MSG(npt, "invalid fib");
 		return (EINVAL);