compat: Add check for DST_NOCACHE
DST_NOCACHE was removed from the Linux 4.13 kernel.  Check if
HAVE_DST_NOCACHE is defined for older kernels.
static inline u32 rt6_get_cookie(const struct rt6_info *rt)
{
if (rt->rt6i_flags & RTF_PCPU ||
+#ifdef HAVE_DST_NOCACHE
(unlikely(rt->dst.flags & DST_NOCACHE) && rt->dst.from))
+#else
+ (unlikely(!list_empty(&rt->rt6i_uncached)) && rt->dst.from))
+#endif
rt = (struct rt6_info *)(rt->dst.from);
return rt->rt6i_node ? rt->rt6i_node->fn_sernum : 0;
