nvme: fix kernel paging oops

mainline inclusion
from mainline-5.0-rc1
commit 092ff0520070fad8407b196f3bb6156ce77a6f98
category: bugfix
bugzilla: 9963
CVE: NA
---------------------------

free the controller discard_page correctly.

Fixes: cb5b7262b011 ("nvme: provide fallback for discard alloc failure")
Signed-off-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NYufen Yu <yuyufen@huawei.com>
Reviewed-by: NMiao Xie <miaoxie@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

drivers/nvme/host/core.c

@@ -3538,7 +3538,7 @@ static void nvme_free_ctrl(struct device *dev)
 	ida_simple_remove(&nvme_instance_ida, ctrl->instance);
 	kfree(ctrl->effects);
 	nvme_mpath_uninit(ctrl);
-	kfree(ctrl->discard_page);
+	__free_page(ctrl->discard_page);
 
 	if (subsys) {
 		mutex_lock(&subsys->lock);