powerpc/kcsan: Exclude udelay to prevent recursive instrumentation

In order for KCSAN to increase its likelihood of observing a data race,
it sets a watchpoint on memory accesses and stalls, allowing for
detection of conflicting accesses by other kernel threads or interrupts.

Stalls are implemented by injecting a call to udelay in instrumented code.
To prevent recursive instrumentation, exclude udelay from being instrumented.
Signed-off-by: Rohan McLure <rmclure@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230206021801.105268-3-rmclure@linux.ibm.com

arch/powerpc/kernel/time.c

@@ -356,7 +356,7 @@ void vtime_flush(struct task_struct *tsk)
 }
 #endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
 
-void __delay(unsigned long loops)
+void __no_kcsan __delay(unsigned long loops)
 {
 	unsigned long start;
 
@@ -377,7 +377,7 @@ void __delay(unsigned long loops)
 }
 EXPORT_SYMBOL(__delay);
 
-void udelay(unsigned long usecs)
+void __no_kcsan udelay(unsigned long usecs)
 {
 	__delay(tb_ticks_per_usec * usecs);
 }