credit2: fix wrong assert in runq_tickle()

credit2: fix wrong assert in runq_tickle()

Since b047f888d489 ("xen: sched: leave CPUs doing tasklet
work alone") a cpu executing a tasklet, is not marked as
idle.

Therefore:
 - avoid asserting that we can't find the idle vcpu running
   on one of them, which is not true,
 - avoid triggering a preemption on them (and add an assert
   checking that).

This fixes a bug identified by OSSTest, in flight 102372
(on ARM, but it's not at all ARM specific), where the
ASSERT() was triggering like this:

(XEN) Xen call trace:
(XEN)    [<0022af78>] sched_credit2.c#runq_tickle+0x3e8/0x61c (PC)
(XEN)    [<0022aedc>] sched_credit2.c#runq_tickle+0x34c/0x61c (LR)
(XEN)    [<0022b644>] sched_credit2.c#csched2_context_saved+0x128/0x1a4
(XEN)    [<0023303c>] context_saved+0x7c/0xa4
(XEN)    [<0024f660>] domain.c#schedule_tail+0x2b4/0x308
(XEN)    [<0024faac>] context_switch+0x80/0x94
(XEN)    [<0022ff48>] schedule.c#schedule+0x76c/0x7ec
(XEN)    [<002338d4>] softirq.c#__do_softirq+0xcc/0xec
(XEN)    [<00233968>] do_softirq+0x18/0x28
(XEN)    [<00261084>] leave_hypervisor_tail+0x58/0x88
(XEN)    [<002649d0>] entry.o#return_to_guest+0xc/0xb8
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 1:
(XEN) Assertion '!is_idle_vcpu(cur->vcpu)' failed at sched_credit2.c:1009
(XEN) ****************************************

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>