xen/blkfront: fix leaking data in shared pages

stable inclusion
from stable-v4.19.251
commit f4a1391185e30c977bfe1648435c152f806211c7
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I5GLXT


CVE: CVE-2022-26365

--------------------------------

commit 2f446ffe upstream.

When allocating pages to be used for shared communication with the
backend always zero them, this avoids leaking unintended data present
on the pages.

This is CVE-2022-26365, part of XSA-403.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: ChenXiaoSong <chenxiaosong2@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>