Better reporting of total/free usable capacity of the cluster (#15230)

The current code uses approximation using a ratio. The approximation 
can skew if we have multiple pools with different disk capacities.

Replace the algorithm with a simpler one which counts data 
disks and ignore parity disks.

cmd/erasure-server-pool.go

@@ -238,6 +238,7 @@ func (z *erasureServerPools) GetRawData(ctx context.Context, volume, file string
 	return nil
 }
 
+// Return the count of disks in each pool
 func (z *erasureServerPools) SetDriveCounts() []int {
 	setDriveCounts := make([]int, len(z.serverPools))
 	for i := range z.serverPools {