Before this change but after:
aea8776a4368ee8a vfs: fix modtimes not updating when writing via cache #4763
When a file was opened read-only the modtime was read from the cached
file. However this modtime wasn't correct leading to an incorrect
result.
This change fixes the definition of `item.IsDirty` to be true only
when the data is dirty. This fixes the problem as a read only file
isn't considered dirty.