kasan: move and hide kasan_save_enable/restore_multi_shot

- Move kasan_save_enable/restore_multi_shot() declarations to
   mm/kasan/kasan.h, as there is no need for them to be visible outside
   of KASAN implementation.

 - Only define and export these functions when KASAN tests are enabled.

 - Move their definitions closer to other test-related code in report.c.

Link: https://lkml.kernel.org/r/6ba637333b78447f027d775f2d55ab1a40f63c99.1646237226.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kasan.h

@@ -267,10 +267,6 @@ static __always_inline bool kasan_check_byte(const void *addr)
 	return true;
 }
 
-
-bool kasan_save_enable_multi_shot(void);
-void kasan_restore_multi_shot(bool enabled);
-
 #else /* CONFIG_KASAN */
 
 static inline slab_flags_t kasan_never_merge(void)