printf: convert test_hashed into macro

This allows the compiler to check the arguments against the __printf()
attribute on __test(). This produces better diagnostics when incorrect
inputs are passed.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202512061600.89CKQ3ag-lkp@intel.com/
Signed-off-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260121-printf-kunit-printf-attr-v3-1-4144f337ec8b@kernel.org
Signed-off-by: Petr Mladek <pmladek@suse.com>

Author: tamird

lib/tests/printf_kunit.c

@@ -266,15 +266,17 @@ hash_pointer(struct kunit *kunittest)
 	KUNIT_EXPECT_MEMNEQ(kunittest, buf, PTR_STR, PTR_WIDTH);
 }
 
-static void
-test_hashed(struct kunit *kunittest, const char *fmt, const void *p)
-{
-	char buf[PLAIN_BUF_SIZE];
-
-	plain_hash_to_buffer(kunittest, p, buf, PLAIN_BUF_SIZE);
-
-	test(buf, fmt, p);
-}
+/*
+ * This is a macro so that the compiler can compare its arguments to the
+ * __printf() attribute on __test(). This cannot be a function with a __printf()
+ * attribute because GCC requires __printf() functions to be variadic.
+ */
+#define test_hashed(kunittest, fmt, p)						\
+	do {									\
+		char buf[PLAIN_BUF_SIZE];					\
+		plain_hash_to_buffer(kunittest, p, buf, PLAIN_BUF_SIZE);	\
+		test(buf, fmt, p);						\
+	} while (0)
 
 /*
  * NULL pointers aren't hashed.