locking/debug: Fix debugfs API return value checks to use IS_ERR()

Update the checking of return values from debugfs_create_file()
and debugfs_create_dir() to use IS_ERR().

Signed-off-by: Atul Kumar Pant <atulpant.linux@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/20230807121834.7438-1-atulpant.linux@gmail.com