staging: android: ion: Cleanup ion_page_pool_alloc_pages

ion_page_pool_alloc_pages calls alloc_pages to allocate pages for page
pools. If alloc_pages return NULL, it will return NULL, or it will
return the pages allocate from alloc_pages. So we can just return
alloc_pages without any judgement.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>