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. On RT kernels this is a potential livelock when the exiting task
preempted the hrtimer soft interrupt.

This only affects hrtimer based timers as Posix CPU timers cannot be
concurrently expired. For CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this is
obviously impossible as the task cannot run task work and exit at the same
time. The CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n (only non-RT) is prevented
because interrupts are disabled.

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.

Fixes: ec8f954a

 ("posix-timers: Use a callback for cancel synchronization on PREEMPT_RT")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/all/20230425183312.862346341@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>