kunit/overflow: Add tests for STACK_FLEX_ARRAY_SIZE() helper

Add a couple of tests for new STACK_FLEX_ARRAY_SIZE() helper.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/c127631a03cdd7f59bfa091b9666a93bf69d0322.1745355442.git.gustavoars@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

Author: GustavoARSilva

lib/tests/overflow_kunit.c

@@ -1210,6 +1210,10 @@ static void DEFINE_FLEX_test(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, __struct_size(empty->array), 0);
 	KUNIT_EXPECT_EQ(test, __member_size(empty->array), 0);
 
+	KUNIT_EXPECT_EQ(test, STACK_FLEX_ARRAY_SIZE(two, array), 2);
+	KUNIT_EXPECT_EQ(test, STACK_FLEX_ARRAY_SIZE(eight, array), 8);
+	KUNIT_EXPECT_EQ(test, STACK_FLEX_ARRAY_SIZE(empty, array), 0);
+
 	/* If __counted_by is not being used, array size will have the on-stack size. */
 	if (!IS_ENABLED(CONFIG_CC_HAS_COUNTED_BY))
 		array_size_override = 2 * sizeof(s16);