net: hns3: fix one incorrect value of page pool info when queried by debugfs

Currently, when user queries page pool info by debugfs command
"cat page_pool_info", the cnt of allocated page for page pool may be
incorrect because of memory inconsistency problem caused by compiler
optimization.

So this patch uses READ_ONCE() to read value of pages_state_hold_cnt to
fix this problem.

Fixes: 850bfb91 ("net: hns3: debugfs add support dumping page pool info")
Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>