sys/stats: return cgroup mem limit, fall back to sysinfo() (#4002)

This is necessary where in certain environments where
cgroup is used to limit memory usage of a container or
a particular process.

GetStats() is used by caching module to figure out the
optimal cacheable size in memory with cgroup limits
what sysinfo reports might not be the right value set
for a given process.

Fixes #4001

pkg/cgroup/others.go

@@ -0,0 +1,19 @@
+// +build !linux
+
+/*
+ * Minio Cloud Storage, (C) 2017 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 cgroup