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 v4.14.283 by commit 72f2f68970f9bdc252d59e119b385a6441b0b155, it was introduced in version v2.6.22 by commit 586759f03e2e9031ac5589912a51a909ed53c30a. For more details please see the references link.