skbuff: Add a function to check if a page belongs to page_pool

Wrap code for checking if a page is a page_pool page into a
function for better readability and ease of reuse.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Reviewed-by: Yunsheng Lin <linyunsheng@huawei.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Mina Almasry <almarsymina@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>