Previously, the VFS layer would try to handle renames more-or-less by
itself, which really only worked for ext2, and even that was only due to
the replace_child kludge existing specifically for this purpose. This
never worked properly for FATFS, since the VFS layer effectively
depended on filesystems having some kind of reference-counting for
inodes, which is something that simply doesn't exist on any FAT variant
we support.
To resolve various issues with the existing scheme, this commit makes
filesystem implementations themselves responsible for the actual rename
operation, while keeping all the existing validation inside the VFS
layer. The only intended behavior change here is that rename operations
should actually properly work on FATFS.
Instead of using a raw `KBuffer` and letting each implementation to
populating the specific flags on its own, we change things so we only
let each FileSystem implementation to validate the flag and its value
but then store it in a HashMap which its key is the flag name and
the value is a special new class called `FileSystemSpecificOption`
which wraps around `AK::Variant<...>`.
This approach has multiple advantages over the previous:
- It allows runtime inspection of what the user has set on a `MountFile`
description for a specific filesystem.
- It ensures accidental overriding of filesystem specific option that
was already set is not possible
- It removes ugly casting of a `KBuffer` contents to a strongly-typed
values. Instead, a strongly-typed `AK::Variant` is used which ensures
we always get a value without doing any casting.
Please note that we have removed support for ASCII string-oriented flags
as there were no actual use cases, and supporting such type would make
`FileSystemSpecificOption` more complicated unnecessarily for now.
Similarly to DevPtsFS, this filesystem is about exposing loop device
nodes easily in /dev/loop, so userspace doesn't need to do anything in
order to use new devices immediately.
