reject mixed drive situations with drives on root disks (#11057)

till now we used to match the inode number of the root
drive and the drive path minio would use, if they match
we knew that its a root disk.

this may not be true in all situations such as running
inside a container environment where the container might
be mounted from a different partition altogether, root
disk detection might fail.

pkg/disk/root_disk.go

@@ -0,0 +1,37 @@
+/*
+ * MinIO Cloud Storage, (C) 2020 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package disk
+
+import "runtime"
+
+// IsRootDisk returns if diskPath belongs to root-disk, i.e the disk mounted at "/"
+func IsRootDisk(diskPath string, rootDisk string) (bool, error) {
+	if runtime.GOOS == "windows" {
+		// On windows this function is not implemented.
+		return false, nil
+	}
+	info, err := GetInfo(diskPath)
+	if err != nil {
+		return false, err
+	}
+	rootInfo, err := GetInfo(rootDisk)
+	if err != nil {
+		return false, err
+	}
+	return SameDisk(info, rootInfo), nil
+}