xen/evtchn: avoid WARN() when unbinding an event channel

In the Linux kernel, the following vulnerability has been resolved: xen/evtchn: avoid WARN() when unbinding an event channel When unbinding a user event channel, the related handler might be called a last time in case the kernel was built with CONFIG_DEBUG_SHIRQ. This might cause a WARN() in the handler. Avoid that by adding an "unbinding" flag to struct user_event which will short circuit the handler.