On driver shutdown device_obj is being freed twice:
1) In vmbus_free_channels()
2) vmbus_device_release() (which is being triggered by device_unregister() in
   vmbus_device_unregister().
This double kfree leads to the following sporadic crash on driver unload:

[   23.469876] general protection fault: 0000 [#1] SMP
[   23.470036] Modules linked in: hv_vmbus(-)
[   23.470036] CPU: 2 PID: 213 Comm: rmmod Not tainted 3.19.0-rc5_bug923184+ #488
[   23.470036] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090006  05/23/2012
[   23.470036] task: ffff880036ef1cb0 ti: ffff880036ce8000 task.ti: ffff880036ce8000
[   23.470036] RIP: 0010:[<ffffffff811d2e1b>]  [<ffffffff811d2e1b>] __kmalloc_node_track_caller+0xdb/0x1e0
[   23.470036] RSP: 0018:ffff880036cebcc8  EFLAGS: 00010246
...

When this crash does not happen on driver unload the similar one is expected if
we try to load hv_vmbus again.

Remove kfree from vmbus_free_channels() as freeing it from
vmbus_device_release() seems right.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>