posix: Fix windows performance issues. (#3132)

Do not attempt to fetch volume/drive information for
each i/o situation. In our case we do this in all calls
`posix.go` this in-turn created a terrible situation for
windows. This issue does not affect the i/o path on Unix
platforms since statvfs calls are in the range of micro
seconds on these platforms.

This verification is only needed during startup and we
let things fail at a later stage on windows.

cmd/posix-errors_test.go

@@ -0,0 +1,57 @@
+/*
+ * Minio Cloud Storage, (C) 2016 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 cmd
+
+import (
+	"os"
+	"runtime"
+	"syscall"
+	"testing"
+)
+
+func TestSysErrors(t *testing.T) {
+	pathErr := &os.PathError{Err: syscall.ENAMETOOLONG}
+	ok := isSysErrTooLong(pathErr)
+	if !ok {
+		t.Fatalf("Unexpected error expecting %s", syscall.ENAMETOOLONG)
+	}
+	pathErr = &os.PathError{Err: syscall.ENOTDIR}
+	ok = isSysErrNotDir(pathErr)
+	if !ok {
+		t.Fatalf("Unexpected error expecting %s", syscall.ENOTDIR)
+	}
+	if runtime.GOOS != "windows" {
+		pathErr = &os.PathError{Err: syscall.ENOTEMPTY}
+		ok = isSysErrNotEmpty(pathErr)
+		if !ok {
+			t.Fatalf("Unexpected error expecting %s", syscall.ENOTEMPTY)
+		}
+	} else {
+		pathErr = &os.PathError{Err: syscall.Errno(0x91)}
+		ok = isSysErrNotEmpty(pathErr)
+		if !ok {
+			t.Fatal("Unexpected error expecting 0x91")
+		}
+	}
+	if runtime.GOOS == "windows" {
+		pathErr = &os.PathError{Err: syscall.Errno(0x03)}
+		ok = isSysErrPathNotFound(pathErr)
+		if !ok {
+			t.Fatal("Unexpected error expecting 0x03")
+		}
+	}
+}