xen: sched: avoid spuriously re-enabling IRQs in csched2_switch_sched()

xen: sched: avoid spuriously re-enabling IRQs in csched2_switch_sched()

interrupts are already disabled when calling the hook
(from schedule_cpu_switch()), so we must use spin_lock()
and spin_unlock().

Add an ASSERT(), so we will notice if this code and its
caller get out of sync with respect to disabling interrupts
(and add one at the same exact occurrence of this pattern
in Credit1 too)

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>