Kernel: Fix Thread destruction after sys_exit

This had undefined behaviour as Thread's (Processes's) PageTable was
destroyed before Thread had the change to destroy its own stacks that
lived on the PageTable.

kernel/include/kernel/Process.h

@@ -47,7 +47,7 @@ namespace Kernel
 		static Process* create_kernel(entry_t, void*);
 		static BAN::ErrorOr<Process*> create_userspace(const Credentials&, BAN::StringView path, BAN::Span<BAN::StringView> arguments);
 		~Process();
-		void cleanup_function();
+		void cleanup_function(Thread*);
 
 		void register_to_scheduler();
 		void exit(int status, int signal);

kernel/kernel/Process.cpp

@@ -208,7 +208,7 @@ namespace Kernel
 	{
 		ASSERT(m_threads.empty());
 		ASSERT(m_mapped_regions.empty());
-		ASSERT(&PageTable::current() != m_page_table.ptr());
+		ASSERT(!m_page_table);
 	}
 
 	void Process::add_thread(Thread* thread)
@@ -217,7 +217,7 @@ namespace Kernel
 		MUST(m_threads.push_back(thread));
 	}
 
-	void Process::cleanup_function()
+	void Process::cleanup_function(Thread* thread)
 	{
 		{
 			SpinLockGuard _(s_process_lock);
@@ -238,6 +238,8 @@ namespace Kernel
 
 		// NOTE: We must unmap ranges while the page table is still alive
 		m_mapped_regions.clear();
+
+		thread->give_keep_alive_page_table(BAN::move(m_page_table));
 	}
 
 	bool Process::on_thread_exit(Thread& thread)

kernel/kernel/Thread.cpp

@@ -242,7 +242,7 @@ namespace Kernel
 
 				ASSERT(thread->m_process == process);
 
-				process->cleanup_function();
+				process->cleanup_function(thread);
 
 				thread->m_delete_process = true;