xen: add helper for calling notifier_call_chain() to common/cpu.c

xen: add helper for calling notifier_call_chain() to common/cpu.c

Add a helper cpu_notifier_call_chain() to call notifier_call_chain()
for a cpu with a specified action, returning an errno value.

This avoids coding the same pattern multiple times.

While at it avoid side effects from using BUG_ON() by not using
cpu_online(cpu) as a parameter.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Jan Beulich <jbeulich@suse.com>