mm: page_frag: avoid caller accessing 'page_frag_cache' directly

Use appropriate frag_page API instead of caller accessing
'page_frag_cache' directly.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Linux-MM <linux-mm@kvack.org>
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20241028115343.3405838-5-linyunsheng@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>