Drivers: hv: vmbus: Fix a bug in hv_need_to_signal()

Drivers: hv: vmbus: Fix a bug in hv_need_to_signal()

commit 288fa3e022eb85fa151e0f9bcd15caeb81679af6 upstream.

As part of updating the vmbus protocol, the function hv_need_to_signal()
was introduced. This functions helps optimize signalling from guest to
host. The newly added memory barrier is needed to ensure that we correctly
decide when to signal the host.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Reported-by: Olaf Hering <olh@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>