ACPI: CPPC: Fix NULL pointer dereference when nosmp is used

With nosmp in cmdline, other CPUs are not brought up, leaving
their cpc_desc_ptr NULL. CPU0's iteration via for_each_possible_cpu()
dereferences these NULL pointers, causing panic.

Panic backtrace:

[    0.401123] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000b8
...
[    0.403255] [<ffffffff809a5818>] cppc_allow_fast_switch+0x6a/0xd4
...
Kernel panic - not syncing: Attempted to kill init!

Fixes: 3cc30dd ("cpufreq: CPPC: Enable fast_switch")
Reported-by: Xu Lu <[email protected]>
Signed-off-by: Yunhui Cui <[email protected]>
Link: https://patch.msgid.link/[email protected]
[ rjw: New subject ]
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: cuiyunhui

drivers/acpi/cppc_acpi.c

@@ -476,7 +476,7 @@ bool cppc_allow_fast_switch(void)
 	struct cpc_desc *cpc_ptr;
 	int cpu;
 
-	for_each_possible_cpu(cpu) {
+	for_each_present_cpu(cpu) {
 		cpc_ptr = per_cpu(cpc_desc_ptr, cpu);
 		desired_reg = &cpc_ptr->cpc_regs[DESIRED_PERF];
 		if (!CPC_IN_SYSTEM_MEMORY(desired_reg) &&