Kernel: Fix triple-fault when clicking on SystemServer in SystemMonitor

The fault was happening when retrieving a current backtrace for the
SystemServer process.

To generate a backtrace, we go into the paging scope of the process,
meaning we temporarily switch to using its page directory as our own.

Because kernel VM is allocated on demand, it's possible for a process's
mappings above the 3GB mark to be out-of-date. Normally this just gets
fixed up transparently by the page fault handler (which simply copies
the PDE from the canonical MM.kernel_page_directory() into the current
process.)

However, if the current kernel *stack* is in a piece of memory that
the backtraced process lacks up-to-date PDE's for, we still get a page
fault, but are unable to handle it, since the CPU wants to push to the
stack as part of calling the page fault handler. So we're screwed and
it's a triple-fault.

Fix this by always updating the kernel VM mappings before switching
into a paging scope. In practical terms, this is a 1KB memcpy() that
happens when generating a backtrace, or doing exec().

Kernel/VM/PageDirectory.h

@@ -31,6 +31,8 @@ public:
     Process* process() { return m_process; }
     const Process* process() const { return m_process; }
 
+    void update_kernel_mappings();
+
 private:
     PageDirectory(Process&, const RangeAllocator* parent_range_allocator);
     explicit PageDirectory(PhysicalAddress);