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.
This is a preparation before we can create a usable mechanism to use
filesystem-specific mount flags.
To keep some compatibility with userland code, LibC and LibCore mount
functions are kept being usable, but now instead of doing an "atomic"
syscall, they do multiple syscalls to perform the complete procedure of
mounting a filesystem.
The FileBackedFileSystem IntrusiveList in the VFS code is now changed to
be protected by a Mutex, because when we mount a new filesystem, we need
to check if a filesystem is already created for a given source_fd so we
do a scan for that OpenFileDescription in that list. If we fail to find
an already-created filesystem we create a new one and register it in the
list if we successfully mounted it. We use a Mutex because we might need
to initiate disk access during the filesystem creation, which will take
other mutexes in other parts of the kernel, therefore making it not
possible to take a spinlock while doing this.
