cpupool: fix shutdown with cpupools with different schedulers

cpupool: fix shutdown with cpupools with different schedulers

trying to shutdown the host when a cpupool exists, has
pCPUs, and has a scheduler different than the Xen's default
one, produces this:

 root@Zhaman:~# xl cpupool-cpu-remove Pool-0 8
 root@Zhaman:~# xl cpupool-create name=\"Pool-1\" sched=\"credit2\"
 Using config file "command line"
 cpupool name:   Pool-1
 scheduler:      credit2
 number of cpus: 0
 root@Zhaman:~# xl cpupool-cpu-add Pool-1 8
 root@Zhaman:~# shutdown -h now

 (XEN) ----[ Xen-4.6-unstable  x86_64  debug=y  Not tainted ]----
 (XEN) CPU:    0
 (XEN) RIP:    e008:[<ffff82d080133bdf>] kill_timer+0x56/0x298
 (XEN) RFLAGS: 0000000000010002   CONTEXT: hypervisor
  ... ... ...
 (XEN) Xen call trace:
 (XEN)    [<ffff82d080133bdf>] kill_timer+0x56/0x298
 (XEN)    [<ffff82d08012233f>] csched_free_pdata+0x9b/0xcf
 (XEN)    [<ffff82d08012c30c>] cpu_schedule_callback+0x64/0x8b
 (XEN)    [<ffff82d08011bc7a>] notifier_call_chain+0x67/0x87
 (XEN)    [<ffff82d08010153e>] cpu_down+0xd9/0x12c
 (XEN)    [<ffff82d080101744>] disable_nonboot_cpus+0x93/0x138
 (XEN)    [<ffff82d0801aa6e7>] enter_state_helper+0xbd/0x365
 (XEN)    [<ffff82d0801061e5>] continue_hypercall_tasklet_handler+0x4a/0xb1
 (XEN)    [<ffff82d080132387>] do_tasklet_work+0x78/0xab
 (XEN)    [<ffff82d0801326bd>] do_tasklet+0x5e/0x8a
 (XEN)    [<ffff82d0801646d2>] idle_loop+0x56/0x6b
  ... ... ...
 (XEN) ****************************************
 (XEN) Panic on CPU 0:
 (XEN) FATAL PAGE FAULT
 (XEN) [error_code=0000]
 (XEN) Faulting linear address: 0000000000000041
 (XEN) ****************************************

The fix is, when tearing down a pCPU, call the free_pdata()
hook from the scheduler of the cpupool the pCPU belongs to,
not always the one from the default scheduler.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>