net_sched: fix unused variables in __gnet_stats_copy_basic_cpu()

Probably not a big deal, but we'd better just use the
one we get in retry loop.

Fixes: commit 22e0f8b9

 ("net: sched: make bstats per cpu and estimator RCU safe")
Reported-by: Joe Perches <joe@perches.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>