posix-timers: Prevent RT livelock in itimer_delete()

In the Linux kernel, the following vulnerability has been resolved:

posix-timers: Prevent RT livelock in itimer_delete()

itimer_delete() 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 which have HAVE_POSIX_CPU_TIMERS_TASK_WORK

enabled.

In that case and on RT kernels the existing task could live lock when

preempting the task which does the timer delivery.

Replace spin_unlock() with an invocation of timer_wait_running() to handle

it the same way as the other retry loops in the posix timer code.