Improve the check of DEBUG_NOTIFIERS feature

This is what DEBUG_NOTIFIERS performs (see kernel/notifier.c):

```
#ifdef CONFIG_DEBUG_NOTIFIERS
		if (unlikely(!func_ptr_is_kernel_text(nb->notifier_call))) {
			WARN(1, "Invalid notifier called!");
			nb = next_nb;
			continue;
		}
#endif
```

CFI can do the same better.

Thanks to @thestinger for the idea.

Refers to #99.

kernel_hardening_checker/checks.py

@@ -127,10 +127,12 @@ def add_kconfig_checks(l, arch):
     l += [KconfigCheck('self_protection', 'kspp', 'DEBUG_VIRTUAL', 'y')]
     l += [KconfigCheck('self_protection', 'kspp', 'DEBUG_SG', 'y')]
     l += [KconfigCheck('self_protection', 'kspp', 'DEBUG_CREDENTIALS', 'y')]
-    l += [KconfigCheck('self_protection', 'kspp', 'DEBUG_NOTIFIERS', 'y')]
     l += [KconfigCheck('self_protection', 'kspp', 'INIT_ON_ALLOC_DEFAULT_ON', 'y')]
     l += [KconfigCheck('self_protection', 'kspp', 'STATIC_USERMODEHELPER', 'y')] # needs userspace support
     l += [KconfigCheck('self_protection', 'kspp', 'SCHED_CORE', 'y')]
+    cfi_clang_is_set = KconfigCheck('self_protection', 'kspp', 'CFI_CLANG', 'y')
+    l += [OR(KconfigCheck('self_protection', 'kspp', 'DEBUG_NOTIFIERS', 'y'),
+             cfi_clang_is_set)]
     l += [OR(KconfigCheck('self_protection', 'kspp', 'SCHED_STACK_END_CHECK', 'y'),
              vmap_stack_is_set)]
     kfence_is_set = KconfigCheck('self_protection', 'kspp', 'KFENCE', 'y')
@@ -196,7 +198,6 @@ def add_kconfig_checks(l, arch):
                   gcc_plugins_support_is_set)]
         l += [KconfigCheck('self_protection', 'kspp', 'RANDOMIZE_KSTACK_OFFSET_DEFAULT', 'y')]
     if arch in ('X86_64', 'ARM64'):
-        cfi_clang_is_set = KconfigCheck('self_protection', 'kspp', 'CFI_CLANG', 'y')
         l += [cfi_clang_is_set]
         l += [AND(KconfigCheck('self_protection', 'kspp', 'CFI_PERMISSIVE', 'is not set'),
                   cfi_clang_is_set)]