Drivers: hv: check vmbus_device_create() return value in vmbus_process_offer()

vmbus_device_create() result is not being checked in vmbus_process_offer() and
it can fail if kzalloc() fails. Add the check and do minor cleanup to avoid
additional duplication of "free_channel(); return;" block.

Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>