disable disk-usage when export is root mount path (#6091)

disk usage crawling is not needed when a tenant
is not sharing the same disk for multiple other
tenants. This PR adds an optimization when we
see a setup uses entire disk, we simply rely on
statvfs() to give us total usage.

This PR also additionally adds low priority
scheduling for usage check routine, such that
other go-routines blocked will be automatically
unblocked and prioritized before usage.

pkg/mountinfo/mountinfo_others.go

@@ -1,7 +1,7 @@
-// +build !linux
+// +build !linux,!windows
 
 /*
- * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ * Minio Cloud Storage, (C) 2017, 2018 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,3 +23,8 @@ package mountinfo
 func CheckCrossDevice(paths []string) error {
 	return nil
 }
+
+// IsLikelyMountPoint determines if a directory is a mountpoint.
+func IsLikelyMountPoint(file string) bool {
+	return false
+}