sched: reorganize cpu_disable_scheduler()

sched: reorganize cpu_disable_scheduler()

The function is called both when we want to remove a cpu
from a cpupool, and during cpu teardown, for suspend or
shutdown. If, however, the boot cpu (cpu 0, most of the
times) is not present in the default cpupool, during
suspend or shutdown, Xen crashes like this:

  root@Zhaman:~# xl cpupool-cpu-remove Pool-0 0
  root@Zhaman:~# shutdown -h now
  (XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Tainted:    C ]----
  ...
  (XEN) Xen call trace:
  (XEN)    [<ffff82d0801238de>] _csched_cpu_pick+0x156/0x61f
  (XEN)    [<ffff82d080123db5>] csched_cpu_pick+0xe/0x10
  (XEN)    [<ffff82d08012de3c>] vcpu_migrate+0x18e/0x321
  (XEN)    [<ffff82d08012e4f8>] cpu_disable_scheduler+0x1cf/0x2ac
  (XEN)    [<ffff82d08018bb8d>] __cpu_disable+0x313/0x36e
  (XEN)    [<ffff82d080101424>] take_cpu_down+0x34/0x3b
  (XEN)    [<ffff82d08013097a>] stopmachine_action+0x70/0x99
  (XEN)    [<ffff82d0801325f0>] do_tasklet_work+0x78/0xab
  (XEN)    [<ffff82d080132926>] do_tasklet+0x5e/0x8a
  (XEN)    [<ffff82d08016478c>] idle_loop+0x56/0x6b
  (XEN)
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 15:
  (XEN) Assertion 'cpu < nr_cpu_ids' failed at ...URCES/xen/xen/xen.git/xen/include/xen/cpumask.h:97
  (XEN) ****************************************

There also are problems when we try to suspend or shutdown
with a cpupool configured with just one cpu (no matter, in
this case, whether that is the boot cpu or not):

  root@Zhaman:~# xl create /etc/xen/test.cfg
  root@Zhaman:~# xl cpupool-migrate test Pool-1
  root@Zhaman:~# xl cpupool-list -c
  Name               CPU list
  Pool-0             0,1,2,3,4,5,6,7,8,9,10,11,13,14,15
  Pool-1             12
  root@Zhaman:~# shutdown -h now
  (XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Tainted:    C ]----
  (XEN) CPU:    12
  ...
  (XEN) Xen call trace:
  (XEN)    [<ffff82d08018bb91>] __cpu_disable+0x317/0x36e
  (XEN)    [<ffff82d080101424>] take_cpu_down+0x34/0x3b
  (XEN)    [<ffff82d08013097a>] stopmachine_action+0x70/0x99
  (XEN)    [<ffff82d0801325f0>] do_tasklet_work+0x78/0xab
  (XEN)    [<ffff82d080132926>] do_tasklet+0x5e/0x8a
  (XEN)    [<ffff82d08016478c>] idle_loop+0x56/0x6b
  (XEN)
  (XEN)
  (XEN) ****************************************
  (XEN) Panic on CPU 12:
  (XEN) Xen BUG at smpboot.c:895
  (XEN) ****************************************

In both cases, the problem is the scheduler not being able
to:
 - move all the vcpus to the boot cpu (as the boot cpu is
   not in the cpupool), in the former;
 - move the vcpus away from a cpu at all (as that is the
   only one cpu in the cpupool), in the latter.

Solution is to distinguish, inside cpu_disable_scheduler(),
the two cases of cpupool manipulation and teardown. For
cpupool manipulation, it is correct to ask the scheduler to
take an action, as pathological situation (like there not
being any cpu in the pool where to send vcpus) are taken
care of (i.e., forbidden!) already. For suspend and shutdown,
we don't want the scheduler to be involved at all, as the
final goal is pretty simple: "send all the vcpus to the
boot cpu ASAP", so we just go for it.

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