kernel: posix-cpu-timers: Cleanup CPU timers before freeing them during exec

In the Linux kernel, the following vulnerability has been resolved: posix-cpu-timers: Cleanup CPU timers before freeing them during exec Commit 55e8c8eb2c7b "posix-cpu-timers: Store a reference to a pid not a task" started looking up tasks by PID when deleting a CPU timer. When a non-leader threa...