Kernel: Fix Process use-after-free in Thread finalization

We leak a ref() onto every user process when constructing them,
either via Process::create_user_process(), or via Process::sys$fork().

This ref() is balanced by a corresponding unref() in
Thread::WaitBlockCondition::finalize().

Since kernel processes don't have a leaked ref() on them, this led to
an extra Process::unref() on kernel processes during finalization.
This happened during every boot, with the `init_stage2` process.

Found by turning off kfree() scrubbing. :^)

Kernel/Process.cpp

@@ -181,6 +181,10 @@ RefPtr<Process> Process::create_user_process(RefPtr<Thread>& first_thread, const
 
     register_new(*process);
     error = 0;
+
+    // NOTE: All user processes have a leaked ref on them. It's balanced by Thread::WaitBlockCondition::finalize().
+    (void)process.leak_ref();
+
     return process;
 }