tty: extract tty_flip_buffer_commit() from tty_flip_buffer_push()

stable inclusion
from stable-v4.19.254
commit f8b10bd4d2a95268490c86a6ddac8d5eb7955150
category: bugfix
bugzilla: https://gitee.com/src-openeuler/kernel/issues/I5A5Y1


CVE: CVE-2022-1462

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

commit 716b1058 upstream.

We will need this new helper in the next patch.

Cc: Hillf Danton <hdanton@sina.com>
Cc: 一只狗 <chennbnbnb@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220707082558.9250-1-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: Wang Weiyang <wangweiyang2@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>