CoW not enabled by default and `inplace_data_updates` option does not exist

[ShawnZhong]

It seems that the documents need to be updated to reflect the change in https://github.com/NVSL/linux-nova/commit/ccf810cfd6e17074b95742dce982d74756c47620.

The document says that there is a module option called inplace_data_updates. https://github.com/NVSL/linux-nova/blob/976a4d1f3d5282863b23aa834e02012167be6ee2/Documentation/filesystems/nova.txt#L82 This option does not exist in the code and CoW is not enabled by default: https://github.com/NVSL/linux-nova/blob/976a4d1f3d5282863b23aa834e02012167be6ee2/fs/nova/super.c#L54-L73

In order to enable CoW, one needs to mount with option -o data_cow, and the message nova: Enable copy-on-write updates should be printed to dmesg

https://github.com/NVSL/linux-nova/blob/976a4d1f3d5282863b23aa834e02012167be6ee2/fs/nova/super.c#L278-L281