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.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>