Use DEFINE_SEQ_ATTRIBUTE macro to simplify the code.

Link: https://lore.kernel.org/r/20200916025022.3992627-1-liushixin2@huawei.comSigned-off-by: Liu Shixin <liushixin2@huawei.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>