Drivers: hv: vmbus: Fix a race condition when unregistering a device

When build with Debug the following crash is sometimes observed:
Call Trace:
 [<ffffffff812b9600>] string+0x40/0x100
 [<ffffffff812bb038>] vsnprintf+0x218/0x5e0
 [<ffffffff810baf7d>] ? trace_hardirqs_off+0xd/0x10
 [<ffffffff812bb4c1>] vscnprintf+0x11/0x30
 [<ffffffff8107a2f0>] vprintk+0xd0/0x5c0
 [<ffffffffa0051ea0>] ? vmbus_process_rescind_offer+0x0/0x110 [hv_vmbus]
 [<ffffffff8155c71c>] printk+0x41/0x45
 [<ffffffffa004ebac>] vmbus_device_unregister+0x2c/0x40 [hv_vmbus]
 [<ffffffffa0051ecb>] vmbus_process_rescind_offer+0x2b/0x110 [hv_vmbus]
...

This happens due to the following race: between 'if (channel->device_obj)' check
in vmbus_process_rescind_offer() and pr_debug() in vmbus_device_unregister() the
device can disappear. Fix the issue by taking an additional reference to the
device before proceeding to vmbus_device_unregister().

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>