dm snapshot: fix crash with transient storage and zero chunk size

stable inclusion
from linux-4.19.192
commit 1ff004c41c8205d7677f7b9d7da238a5d9a29274

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

commit c699a0db upstream.

The following commands will crash the kernel:

modprobe brd rd_size=1048576
dmsetup create o --table "0 `blockdev --getsize /dev/ram0` snapshot-origin /dev/ram0"
dmsetup create s --table "0 `blockdev --getsize /dev/ram0` snapshot /dev/ram0 /dev/ram1 N 0"

The reason is that when we test for zero chunk size, we jump to the label
bad_read_metadata without setting the "r" variable. The function
snapshot_ctr destroys all the structures and then exits with "r == 0". The
kernel then crashes because it falsely believes that snapshot_ctr
succeeded.

In order to fix the bug, we set the variable "r" to -EINVAL.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>