networkd: don't try to turn on ipv6 forwarding if kernel lacks IPv6 support

http://lists.freedesktop.org/archives/systemd-devel/2015-May/031598.html

src/network/networkd-link.c

@@ -26,6 +26,7 @@
 #include "util.h"
 #include "virt.h"
 #include "fileio.h"
+#include "socket-util.h"
 #include "bus-util.h"
 #include "udev-util.h"
 #include "network-internal.h"
@@ -1489,6 +1490,10 @@ static int link_set_ipv6_forward(Link *link) {
         const char *p = NULL;
         int r;
 
+        /* Make this a NOP if IPv6 is not available */
+        if (!socket_ipv6_is_supported())
+                return 0;
+
         p = strjoina("/proc/sys/net/ipv6/conf/", link->ifname, "/forwarding");
         r = write_string_file_no_create(p, one_zero(link_ipv6_forward_enabled(link)));
         if (r < 0)