powerpc/smp: Increase nr_cpu_ids to include the boot CPU

If nr_cpu_ids is too low to include the boot CPU adjust nr_cpu_ids
upward. Otherwise the kernel will BUG when trying to allocate a paca
for the boot CPU and fail to boot.

Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231229120107.2281153-2-mpe@ellerman.id.au

arch/powerpc/kernel/prom.c

@@ -381,6 +381,12 @@ static int __init early_init_dt_scan_cpus(unsigned long node,
 			nr_cpu_ids);
 	}
 
+	if (boot_cpuid >= nr_cpu_ids) {
+		set_nr_cpu_ids(min(CONFIG_NR_CPUS, ALIGN(boot_cpuid + 1, nthreads)));
+		pr_warn("Boot CPU %d >= nr_cpu_ids, adjusted nr_cpu_ids to %d\n",
+			boot_cpuid, nr_cpu_ids);
+	}
+
 	/*
 	 * PAPR defines "logical" PVR values for cpus that
 	 * meet various levels of the architecture: