memcg: Uncharge pages outside memcg_stock.lock in refill_stock().

[memcg: Uncharge pages outside memcg_stock.lock in refill_stock().] On host w/ AMD EPYC 9B45 128-Core Processor (Zen 5 Turin, 2 sockets, 512 vCPUs), tcp_rr w/ 20000 flows throughput often degrades by