scsi: storvsc: Payload buffer incorrectly sized for 32 bit kernels.

On a 32 bit kernel sizeof(void *) is not 64 bits as hv_mpb_array
requires. Also the buffer needs to be cleared or the upper bytes will
contain junk.

Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Cathy Avery <cavery@redhat.com>
Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>