sched: fix resuming from S3 with smt=0

sched: fix resuming from S3 with smt=0

When resuming from S3 and smt=0 or maxcpus= are specified we must not
do anything in cpu_schedule_callback(). This is not true today for
taking down a cpu during resume.

If anything goes wrong during resume all the scheduler related error
handling is in cpupool.c, so we can just bail out early from
cpu_schedule_callback() when suspending or resuming.

This fixes commit 0763cd2687897b55e7 ("xen/sched: don't disable
scheduler on cpus during suspend").

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>