percpu: use pcpu_free_slot instead of pcpu_nr_slots - 1

mainline inclusion
from mainline-v5.14-rc1
commit 1c29a3ce
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4BE79


CVE: NA

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This prepares for adding a to_depopulate list and sidelined list after
the free slot in the set of lists in pcpu_slot.

Signed-off-by: Dennis Zhou <dennis@kernel.org>
Acked-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
(cherry picked from commit 1c29a3ce)
Signed-off-by: Yuanzheng Song <songyuanzheng@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>