KASAN: Implement __asan_unregister_globals()

It will be called during KLD unload to unpoison the redzones following
global variables.  Otherwise, virtual address ranges previously used for
a KLD may be left tainted, triggering false positives when they are
recycled.

Reported by:	pho
Sponsored by:	The FreeBSD Foundation

sys/kern/subr_asan.c

@@ -948,7 +948,12 @@ __asan_register_globals(struct __asan_global *globals, size_t n)
 void
 __asan_unregister_globals(struct __asan_global *globals, size_t n)
 {
-	/* never called */
+	size_t i;
+
+	for (i = 0; i < n; i++) {
+		kasan_mark(globals[i].beg, globals[i].size_with_redzone,
+		    globals[i].size_with_redzone, 0);
+	}
 }
 
 #define ASAN_LOAD_STORE(size)					\