[kernel] Protect process::m_threads with a lock

Another spot I meant to go back and clean up with a lock - found it when
a process with threads running on two CPUs exited, and the scheduler
tried to delete the process on both CPUs.

src/kernel/objects/process.h

@@ -111,6 +111,7 @@ private:
     vm_space m_space;
 
     util::vector<thread*> m_threads;
+    util::spinlock m_threads_lock;
 
     util::node_set<j6_handle_t, j6_handle_invalid, heap_allocated> m_handles;
     util::spinlock m_handles_lock;