page_pool: use relaxed atomic for release side accounting

There is no need to synchronize the account updating, so
use the relaxed atomic to avoid some memory barrier in the
data path.

Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>