Immutable mounts are mounts that can't be changed in any aspect, if the
VFSRootContext that hold them is used by a process. This includes two
operations on a mount:
1. Trying to remove the mount from the mount table.
2. Trying to change the flags of the mount.
The condition of a VFSRootContext being held by a process or not is
crucial, as the intention is to allow removal of mounts that marked as
immutable if the VFSRootContext is not being used anymore (for example,
if the container that was created with such context stopped).
Marking mounts as immutable on the first VFS root context essentially
ensures they will never be modified because there will be a process
using that context (which is the "main" VFS root context in the system
runtime).
It should be noted that setting a mount as immutable can be done in
creation time of the mount by passing the MS_IMMUTABLE flag, or by doing
a remount with MS_IMMUTABLE flag.
There's no point in constructing an object just for the sake of keeping
a state that can be touched by anything in the kernel code.
Let's reduce everything to be in a C++ namespace called with the
previous name "VirtualFileSystem" and keep a smaller textual-footprint
struct called "VirtualFileSystemDetails".
This change also cleans up old "friend class" statements that were no
longer needed, and move methods from the VirtualFileSystem code to more
appropriate places as well.
Please note that the method of locking all filesystems during shutdown
is removed, as in that place there's no meaning to actually locking all
filesystems because of running in kernel mode entirely.
The VFSRootContext class, as its name suggests, holds a context for a
root directory with its mount table and the root custody/inode in the
same class.
The idea is derived from the Linux mount namespace mechanism.
It mimicks the concept of the ProcessList object, but it is adjusted for
a root directory tree context.
In contrast to the ProcessList concept, processes that share the default
VFSRootContext can't see other VFSRootContext related properties such as
as the mount table and root custody/inode.
To accommodate to this change progressively, we internally create 2 main
VFS root contexts for now - one for kernel processes (as they don't need
to care about VFS root contexts for the most part), and another for all
userspace programs.
This separation allows us to continue pretending for userspace that
everything is "normal" as it is used to be, until we introduce proper
interfaces in the mount-related syscalls as well as in the SysFS.
We make VFSRootContext objects being listed, as another preparation
before we could expose interfaces to userspace.
As a result, the PowerStateSwitchTask now iterates on all contexts
and tear them down one by one.
