Drivers: hv: vmbus: Close timing hole that can corrupt per-cpu page

commit 13b9abfc

 upstream.

Extend the disabling of preemption to include the hypercall so that
another thread can't get the CPU and corrupt the per-cpu page used
for hypercall arguments.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>