Kernel: Thread::wait_on() must always leave interrupts enabled on exit

The short-circuit path added for waiting on a queue that already had a
pending wake was able to return with interrupts disabled, which breaks
the API contract of wait_on() always returning with IF=1.

Fix this by adding a way to override the restored IF in ScopedCritical.

Kernel/Arch/i386/CPU.h

@@ -837,6 +837,14 @@ public:
         return *this;
     }
 
+    void set_interrupt_flag_on_destruction(bool flag)
+    {
+        if (flag)
+            m_prev_flags |= 0x200;
+        else
+            m_prev_flags &= ~0x200;
+    }
+
 private:
     u32 m_prev_flags { 0 };
     bool m_valid { false };

Kernel/Thread.cpp

@@ -876,6 +876,11 @@ Thread::BlockResult Thread::wait_on(WaitQueue& queue, const char* reason, timeva
                 // The WaitQueue was already requested to wake someone when
                 // nobody was waiting. So return right away as we shouldn't
                 // be waiting
+
+                // The API contract guarantees we return with interrupts enabled,
+                // regardless of how we got called
+                critical.set_interrupt_flag_on_destruction(true);
+
                 return BlockResult::NotBlocked;
             }