staging: hv: Fixed bounce kmap problem by using correct index

Fixed bounce offset kmap problem by using correct index.
The symptom of the problem is that in some NAS appliances this problem
represents Itself by a unresponsive VM under a load with many clients writing
small files.

Signed-off-by:Hank Janssen <hjanssen@microsoft.com>
Signed-off-by:Haiyang Zhang <haiyangz@microsoft.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>