net: dst: move cpu inside ifdef to avoid compilation warning

If CONFIG_DST_CACHE is not selected cpu variable
will be unused and we will see a compilation warning.
Move it under the ifdef.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: d66f2b91f95b ("bpf: don't rely on the verifier lock for metadata_dst allocation")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -325,9 +325,9 @@ EXPORT_SYMBOL_GPL(metadata_dst_alloc_percpu);
 
 void metadata_dst_free_percpu(struct metadata_dst __percpu *md_dst)
 {
+#ifdef CONFIG_DST_CACHE
 	int cpu;
 
-#ifdef CONFIG_DST_CACHE
 	for_each_possible_cpu(cpu) {
 		struct metadata_dst *one_md_dst = per_cpu_ptr(md_dst, cpu);