powerpc/64: Fix task_cpu in early boot when booting non-zero cpuid

powerpc/64 can boot on a non-zero SMP processor id. Initially, the boot
CPU is said to be "assumed to be 0" until early_init_devtree() discovers
the id from the device tree. That is not a good description because the
assumption can be wrong and that has to be handled, the better
description is that 0 is used as a placeholder, and things are fixed
after the real id is discovered.

smp_processor_id() is set to the boot cpuid, but task_cpu(current) is
not, which causes the smp_processor_id() == task_cpu(current) invariant
to be broken until init_idle() in sched_init().

This is quite fragile and could lead to subtle bugs in future. One bug
is that validate_sp_size uses task_cpu() to get the process stack, so
any stack trace from the booting CPU between early_init_devtree()
and sched_init() will have problems. Early on paca_ptrs[0] will be
poisoned, so that can cause machine checks dereferencing that memory
in real mode. Later, validating the current stack pointer against the
idle task of a different secondary will probably cause no stack trace
to be printed.

Fix this by setting thread_info->cpu right after smp_processor_id() is
set to the boot cpuid.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Fix SMP=n build as reported by sfr]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221216115930.2667772-3-npiggin@gmail.com

arch/powerpc/kernel/setup_64.c

@@ -396,6 +396,11 @@ void __init early_setup(unsigned long dt_ptr)
 	}
 	fixup_boot_paca(paca_ptrs[boot_cpuid]);
 	setup_paca(paca_ptrs[boot_cpuid]); /* install the paca into registers */
+	// smp_processor_id() now reports boot_cpuid
+
+#ifdef CONFIG_SMP
+	task_thread_info(current)->cpu = boot_cpuid; // fix task_cpu(current)
+#endif
 
 	/*
 	 * Configure exception handlers. This include setting up trampolines