vfio/fsl-mc: Block calling interrupt handler without trigger

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

vfio/fsl-mc: Block calling interrupt handler without trigger

The eventfd_ctx trigger pointer of the vfio_fsl_mc_irq object is

initially NULL and may become NULL if the user sets the trigger

eventfd to -1. The interrupt handler itself is guaranteed that

trigger is always valid between request_irq() and free_irq(), but

the loopback testing mechanisms to invoke the handler function

need to test the trigger. The triggering and setting ioctl paths

both make use of igate and are therefore mutually exclusive.

The vfio-fsl-mc driver does not make use of irqfds, nor does it

support any sort of masking operations, therefore unlike vfio-pci

and vfio-platform, the flow can remain essentially unchanged.