xen: privcmd: Switch from mutex to spinlock for irqfds

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

xen: privcmd: Switch from mutex to spinlock for irqfds

irqfd_wakeup() gets EPOLLHUP, when it is called by

eventfd_release() by way of wake_up_poll(&ctx->wqh, EPOLLHUP), which

gets called under spin_lock_irqsave(). We can't use a mutex here as it

will lead to a deadlock.

Fix it by switching over to a spin lock.