dlm: fix plock invalid read
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v5.4.198 by commit 49cd9eb7b9a7b88124b31e31f8e539acaf1b3a6d, it was introduced in version v2.6.22 by commit 586759f03e2e9031ac5589912a51a909ed53c30a. For more details please see the references link.