powerpc/kcsan: Properly instrument arch_spin_unlock()

The following boottime error is encountered with SMP kernel:

  kcsan: improperly instrumented type=(0): arch_spin_unlock(&arch_spinlock)
  kcsan: improperly instrumented type=(0): spin_unlock(&test_spinlock)
  kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE): arch_spin_unlock(&arch_spinlock)
  kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE): spin_unlock(&test_spinlock)
  kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND): arch_spin_unlock(&arch_spinlock)
  kcsan: improperly instrumented type=(KCSAN_ACCESS_WRITE | KCSAN_ACCESS_COMPOUND): spin_unlock(&test_spinlock)
  kcsan: selftest: test_barrier failed
  kcsan: selftest: 2/3 tests passed
  Kernel panic - not syncing: selftests failed

Properly instrument arch_spin_unlock() with kcsan_mb().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/57834a703dfa5d6c27c9de0a01329059636e5ab7.1685080579.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/simple_spinlock.h

@@ -15,6 +15,7 @@
  * (the type definitions are in asm/simple_spinlock_types.h)
  */
 #include <linux/irqflags.h>
+#include <linux/kcsan-checks.h>
 #include <asm/paravirt.h>
 #include <asm/paca.h>
 #include <asm/synch.h>
@@ -126,6 +127,7 @@ static inline void arch_spin_lock(arch_spinlock_t *lock)
 
 static inline void arch_spin_unlock(arch_spinlock_t *lock)
 {
+	kcsan_mb();
 	__asm__ __volatile__("# arch_spin_unlock\n\t"
 				PPC_RELEASE_BARRIER: : :"memory");
 	lock->slock = 0;