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