smp: Fix a potential usage of stale nr_cpus

The get_option() maybe return 0, it means that the nr_cpus is
not initialized. Then we will use the stale nr_cpus to initialize
the nr_cpu_ids. So fix it.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200716070457.53255-1-songmuchun@bytedance.com