lib: fix the NULL vs IS_ERR() bug for debugfs_create_dir()

debugfs_create_dir() returns error pointers.  It never returns NULL.  So
use IS_ERR() to check it.

Link: https://lkml.kernel.org/r/20240821073441.9701-1-11162571@vivo.comSigned-off-by: Yang Ruibin <11162571@vivo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/test_fpu_glue.c

@@ -42,7 +42,7 @@ static int __init test_fpu_init(void)
 		return -EINVAL;
 
 	selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
-	if (!selftest_dir)
+	if (IS_ERR(selftest_dir))
 		return -ENOMEM;
 
 	debugfs_create_file_unsafe("test_fpu", 0444, selftest_dir, NULL,