fscommon.WriteFile: simplify error message

As the underlying error message from iotuils.WriteFile already contains
file name, there's no need to put it, otherwise we end up with something
like:

	failed to write "val" to "/sys/fs/cgroup/.../file": open /sys/fs/cgroup/.../file: permission denied

With this patch, the error will be

	failed to write "val": open /sys/fs/cgroup/.../file: permission denied

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/cgroups/fscommon/fscommon.go

@@ -21,7 +21,7 @@ func WriteFile(dir, file, data string) error {
 		return err
 	}
 	if err := retryingWriteFile(path, []byte(data), 0700); err != nil {
-		return errors.Wrapf(err, "failed to write %q to %q", data, path)
+		return errors.Wrapf(err, "failed to write %q", data)
 	}
 	return nil
 }