When channel->device_obj is non-NULL, vmbus_onoffer_rescind() could
invoke put_device(), that will eventually release the device and free
the channel object (cf. vmbus_device_release()).  However, a pointer
to the object is dereferenced again later to load the primary_channel.
The use-after-free can be avoided by noticing that this load/check is
redundant if device_obj is non-NULL: primary_channel must be NULL if
device_obj is non-NULL, cf. vmbus_add_channel_work().

Fixes: 54a66265 ("Drivers: hv: vmbus: Fix rescind handling")
Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20201209070827.29335-5-parri.andrea@gmail.comSigned-off-by: Wei Liu <wei.liu@kernel.org>