mm/dmapool: add MODULE_DESCRIPTION()

Fix the 'make W=1' warning:

WARNING: modpost: missing MODULE_DESCRIPTION() in mm/dmapool_test.o

Link: https://lkml.kernel.org/r/20240513-mm-md-v1-2-8c20e7d26842@quicinc.com


Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>