ptp: unregister virtual clocks when unregistering physical clock.

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

ptp: unregister virtual clocks when unregistering physical clock.

When unregistering a physical clock which has some virtual clocks,

unregister the virtual clocks with it.

This fixes the following oops, which can be triggered by unloading

a driver providing a PTP clock when it has enabled virtual clocks:

BUG: unable to handle page fault for address: ffffffffc04fc4d8

Oops: 0000 [#1] PREEMPT SMP NOPTI

RIP: 0010:ptp_vclock_read+0x31/0xb0

Call Trace:

timecounter_read+0xf/0x50

ptp_vclock_refresh+0x2c/0x50

? ptp_clock_release+0x40/0x40

ptp_aux_kworker+0x17/0x30

kthread_worker_fn+0x9b/0x240

? kthread_should_park+0x30/0x30

kthread+0xe2/0x110

? kthread_complete_and_exit+0x20/0x20

ret_from_fork+0x22/0x30