lib/on-cpus: Fix on_cpumask

on_cpumask should wait until the cpus in the mask, not including
the calling cpu, are idle. Checking the weight against nr_cpus
minus 1 only works when the mask is the same as the present mask.

Fixes: d012cfd5 ("lib/on-cpus: Introduce on_cpumask and on_cpumask_async")
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>