staging: erofs: refuse to mount images with malformed volume name

staging: erofs: refuse to mount images with malformed volume name

As Richard reminder [1], A valid volume name should be
ended in NIL terminator within the length of volume_name.

Since this field currently isn't really used, let's fix
it to avoid potential bugs in the future.

[1] https://lore.kernel.org/r/1133002215.69049.1566119033047.JavaMail.zimbra@nod.at/

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190818102824.22330-1-hsiangkao@aol.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>