mm/dmapool: use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO() helper instead of plain DEVICE_ATTR(), which makes
the code a bit shorter and easier to read.

Link: https://lkml.kernel.org/r/20210524112852.34716-1-yuehaibing@huawei.com


Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>