scsi: storvsc: Reduce default ring buffer size to 128 Kbytes

Reduce the default VMbus channel ring buffer size for storvsc SCSI devices
from 1 Mbyte to 128 Kbytes. Measurements show that ring buffer sizes above
128 Kbytes do not increase performance even at very high IOPS rates, so
don't waste the memory. Also remove the dependence on PAGE_SIZE, since the
ring buffer size should not change on architectures where PAGE_SIZE is not
4 Kbytes.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>