powerpc/pseries: unregister VPA when hot unplugging a CPU

The VPA should unregister when offlining a CPU. Otherwise there could be
a short window where 2 CPUs could share the same VPA.

This happens because the hypervisor is still keeping the VPA attached to
the vCPU even if it became offline.

Here is a potential situation:
 1. remove proc A,
 2. add proc B. If proc B gets proc A's place in cpu_present_mask, then
    it registers proc A's VPAs.
 3. If proc B is then re-added to the LP, its threads are sharing VPAs
    with proc A briefly as they come online.

As the hypervisor may check for the VPA's yield_count field oddity, it
may detect an unexpected value and kill the LPAR.

Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
[mpe: s/cpu_present_map/cpu_present_mask/ in change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221114160150.13554-1-ldufour@linux.ibm.com

arch/powerpc/platforms/pseries/hotplug-cpu.c

@@ -70,6 +70,7 @@ static void pseries_cpu_offline_self(void)
 		xics_teardown_cpu();
 
 	unregister_slb_shadow(hwcpu);
+	unregister_vpa(hwcpu);
 	rtas_stop_self();
 
 	/* Should never get here... */