ring_buffer: Do not deactivate non-existant pages

stable inclusion
from stable-v4.19.267
commit 455ea324770205525cbc13f155806a5346794339
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I63UEU
CVE: NA

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

commit 56f4ca0a upstream.

rb_head_page_deactivate() expects cpu_buffer to contain a valid list of
->pages, so verify that the list is actually present before calling it.

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Link: https://lkml.kernel.org/r/20221114143129.3534443-1-d-tatianin@yandex-team.ru



Cc: stable@vger.kernel.org
Fixes: 77ae365e ("ring-buffer: make lockless")
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>