CVE-2023-53815 posix-timers: Prevent RT livelock in itimer_delete()

In the Linux kernel, the following vulnerability has been resolved: posix-timers: Prevent RT livelock in itimerdelete itimerdelete has a retry loop when the timer is concurrently expired. On non-RT kernels this just spin-waits until the timer callback has completed, except for posix CPU timers...