kunit: kasan_test: disable fortify string checker on kasan_strings() test

Similar to commit 09c6304 ("kasan: test: fix compatibility with
FORTIFY_SOURCE") the kernel is panicing in kasan_string().

This is due to the `src` and `ptr` not being hidden from the optimizer
which would disable the runtime fortify string checker.

Call trace:
  __fortify_panic+0x10/0x20 (P)
  kasan_strings+0x980/0x9b0
  kunit_try_run_case+0x68/0x190
  kunit_generic_run_threadfn_adapter+0x34/0x68
  kthread+0x1c4/0x228
  ret_from_fork+0x10/0x20
 Code: d503233f a9bf7bfd 910003fd 9424b243 (d4210000)
 ---[ end trace 0000000000000000 ]---
 note: kunit_try_catch[128] exited with irqs disabled
 note: kunit_try_catch[128] exited with preempt_count 1
     # kasan_strings: try faulted: last
** replaying previous printk message **
     # kasan_strings: try faulted: last line seen mm/kasan/kasan_test_c.c:1600
     # kasan_strings: internal error occurred preventing test case from running: -4

Link: https://lkml.kernel.org/r/20250801120236.2962642-1-yeoreum.yun@arm.com
Fixes: 73228c7 ("KASAN: port KASAN Tests to KUnit")
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitriy Vyukov <dvyukov@google.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: LeviYeoReum

mm/kasan/kasan_test_c.c

@@ -1578,9 +1578,11 @@ static void kasan_strings(struct kunit *test)
 
 	ptr = kmalloc(size, GFP_KERNEL | __GFP_ZERO);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
+	OPTIMIZER_HIDE_VAR(ptr);
 
 	src = kmalloc(KASAN_GRANULE_SIZE, GFP_KERNEL | __GFP_ZERO);
 	strscpy(src, "f0cacc1a0000000", KASAN_GRANULE_SIZE);
+	OPTIMIZER_HIDE_VAR(src);
 
 	/*
 	 * Make sure that strscpy() does not trigger KASAN if it overreads into