Currently, page->pp is cleared and set everytime the page
is recycled, which is unnecessary.

So only set the page->pp when the page is added to the page
pool and only clear it when the page is released from the
page pool.

This is also a preparation to support allocating frag page
in page pool.
Reviewed-by: NIlias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: NYunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>