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.