sched: get rid of cpupool_scheduler_cpumask()

sched: get rid of cpupool_scheduler_cpumask()

and of (almost every) direct use of cpupool_online_cpumask().

In fact, what we really want for the most of the times,
is the set of valid pCPUs of the cpupool a certain domain
is part of. Furthermore, in case it's called with a NULL
pool as argument, cpupool_scheduler_cpumask() does more
harm than good, by returning the bitmask of free pCPUs!

This commit, therefore:
 * gets rid of cpupool_scheduler_cpumask(), in favour of
   cpupool_domain_cpumask(), which makes it more evident
   what we are after, and accommodates some sanity checking;
 * replaces some of the calls to cpupool_online_cpumask()
   with calls to the new functions too.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Juergen Gross <jgross@suse.com>
Acked-by: Joshua Whitehead <josh.whitehead@dornerworks.com>
Reviewed-by: Meng Xu <mengxu@cis.upenn.edu>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>