Netvsc has a function to calculate how much ring buffer in percentage is
available to write. This function is also useful for storvsc and other
vmbus devices.

Define a similar function in vmbus to be used by other vmbus devices.
Signed-off-by: NLong Li <longli@microsoft.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>