watchdog/hardlockup/perf: Warn if watchdog_ev is leaked

When creating a new perf_event for the hardlockup watchdog, it should not
happen that the old perf_event is not released.

Introduce a WARN_ONCE() that should never trigger.

[ mingo: Changed the type of the warning to WARN_ONCE(). ]

Signed-off-by: Li Huafei <lihuafei1@huawei.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241021193004.308303-2-lihuafei1@huawei.com