We don't really need such a big on-stack buffer.  vmbus_sendpacket() here
only uses sizeof(struct pci_child_message).
Signed-off-by: NDexuan Cui <decui@microsoft.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NJake Oshins <jakeo@microsoft.com>