Kernel/Process: Move protected values to the end of the object

The compiler can re-order the structure (class) members if that's
necessary, so if we make Process to inherit from ProcFSExposedComponent,
even if the declaration is to inherit first from ProcessBase, then from
ProcFSExposedComponent and last from Weakable<Process>, the members of
class ProcFSExposedComponent (including the Ref-counted parts) are the
first members of the Process class.

This problem made it impossible to safely use the current toggling
method with the write-protection bit on the ProcessBase members, so
instead of inheriting from it, we make its members the last ones in the
Process class so we can safely locate and modify the corresponding page
write protection bit of these values.

We make sure that the Process class doesn't expand beyond 8192 bytes and
the protected values are always aligned on a page boundary.

Kernel/Syscalls/setpgid.cpp

@@ -40,7 +40,7 @@ KResultOr<FlatPtr> Process::sys$setsid()
     m_pg = ProcessGroup::create(ProcessGroupID(pid().value()));
     m_tty = nullptr;
     ProtectedDataMutationScope scope { *this };
-    m_sid = pid().value();
+    m_protected_values.sid = pid().value();
     return sid().value();
 }