smp: don't declare nr_cpu_ids if NR_CPUS == 1

mainline inclusion
from mainline-v6.1-rc1
commit 53fc190c
category: cleanup
bugzilla: https://gitee.com/openeuler/kernel/issues/I8KQBZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=53fc190cc6771c5494d782210334d4ebb50c7103



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SMP and NR_CPUS are independent options, hence nr_cpu_ids may be
declared even if NR_CPUS == 1, which is useless.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Zheng Zucheng <zhengzucheng@huawei.com>