Incorrect error message for insufficient volume fix (#6099)

Reply back with appropriate error message when the server is spawn
with volume of insufficient size (< 1GiB).

Fixes #5993.

cmd/posix.go

@@ -224,7 +224,7 @@ func checkDiskMinTotal(di disk.Info) (err error) {
 	// used for journalling, inodes etc.
 	totalDiskSpace := float64(di.Total) * 0.95
 	if int64(totalDiskSpace) <= diskMinTotalSpace {
-		return errDiskFull
+		return errMinDiskSize
 	}
 	return nil
 }