net: dst_cache: minor optimization in dst_cache_set_ip6()

There is no need to use this_cpu_ptr(dst_cache->cache) twice.

Compiler is unable to optimize the second call, because of
per-cpu constraints.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240507132717.627518-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dst_cache.c

@@ -112,7 +112,7 @@ void dst_cache_set_ip6(struct dst_cache *dst_cache, struct dst_entry *dst,
 		return;
 
 	idst = this_cpu_ptr(dst_cache->cache);
-	dst_cache_per_cpu_dst_set(this_cpu_ptr(dst_cache->cache), dst,
+	dst_cache_per_cpu_dst_set(idst, dst,
 				  rt6_get_cookie(dst_rt6_info(dst)));
 	idst->in6_saddr = *saddr;
 }