pps: Fix a use-after free bug when unregistering a source.
Remove the cdev from the system (with cdev_del) *before* deallocating it
(in pps_device_destruct, called via kobject_put from device_destroy).
Also prevent deallocating a device with open file handles.
A better long-term fix is probably to remove the cdev from the pps_device
entirely, and instead have all devices reference one global cdev. Then
the deallocation ordering becomes simpler.
But that's more complex and invasive change, so we leave that
for later.