net: page_pool: factor out releasing DMA from releasing the page

Releasing the DMA mapping will be useful for other types
of pages, so factor it out. Make sure compiler inlines it,
to avoid any regressions.

Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>