net: page_pool: fix kabi issue for page poos statistics

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8BNYE


CVE: NA

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Currently the page pool statistisc member is place at
middle part of struct page_pool, which may break the
kabi. To fix it, replaced the KABI_RESERVE(1) field
with a pointer refer to the page pool statistics.

Signed-off-by: Jian Shen <shenjian15@huawei.com>