In Vmbus, we have defined a function to calculate available ring buffer
percentage to write.

Use that function and remove netvsc's private version.

[mkp: typo]
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>