Remove src-prefix when ipv6_subtrees is disabled

Note that the previous auto-detection is wrong. IPV6_SUBTREES exists
since (at least?) Linux 2.6.32. On the other side, a recent kernel may
be built without source-based routing.

Our use of this patch is safe because we control entirely with
re6stnet which routes babeld is going to install.

babeld.man

@@ -196,9 +196,8 @@ equivalent to the command-line option
 .BR \-r .
 .TP
 .BR ipv6-subtrees " {" true | false }
-This specifies whether to use native source-specific IPv6 forwarding
-rather than multiple routing tables.  The default is chosen automatically
-depending on the kernel version.
+Linux IPv6 only, default is true. If false, source prefix is cleared when the
+route is installed. Do not use unless you know what you're doing.
 .TP
 .BI debug " level"
 This specifies the debugging level, and is equivalent to the command-line

kernel_netlink.c

@@ -999,7 +999,7 @@ kernel_interface_channel(const char *ifname, int ifindex)
 int
 kernel_has_ipv6_subtrees(void)
 {
-    return (kernel_older_than("Linux", 3, 11) == 0);
+    return 1;
 }
 
 int
@@ -1095,10 +1095,15 @@ kernel_route(int operation, int table,
     is_v4_over_v6 = ipv4 && !v4mapped(gate);
     use_src = !is_default(src, src_plen);
     if(use_src) {
-        if(ipv4 || !has_ipv6_subtrees) {
+        if(ipv4) {
             errno = ENOSYS;
             return -1;
         }
+        /* WARNING: There's no autodetection for the calculation of
+         *          has_ipv6_subtrees: it's 1 unless explicitly configured
+         *          with 'ipv6-subtrees false', which should only be used
+         *          if it's safe to ignore the source prefix. */
+        use_src = has_ipv6_subtrees;
     }
 
     kdebugf("kernel_route: %s %s from %s "