DoublyLinkedList: Add a RemoveAllBefore convenience function.

Extracted from hamishm's event queue patches.

headers/private/kernel/util/DoublyLinkedList.h

@@ -338,6 +338,8 @@ public:
 	inline void Add(Element* element, bool back = true);
 	inline void Remove(Element* element);
 
+	inline void RemoveAllBefore(Element* element);
+
 	inline void Swap(Element* a, Element* b);
 
 	inline void MoveFrom(DOUBLY_LINKED_LIST_CLASS_NAME* fromList);
@@ -500,6 +502,7 @@ DOUBLY_LINKED_LIST_CLASS_NAME::Add(Element* element, bool back)
 	Insert(element, back);
 }
 
+
 // Remove
 DOUBLY_LINKED_LIST_TEMPLATE_LIST
 void
@@ -528,6 +531,19 @@ DOUBLY_LINKED_LIST_CLASS_NAME::Remove(Element* element)
 	}
 }
 
+
+// RemoveAllBefore
+DOUBLY_LINKED_LIST_TEMPLATE_LIST
+void
+DOUBLY_LINKED_LIST_CLASS_NAME::RemoveAllBefore(Element* element)
+{
+	if (element != NULL) {
+		fFirst = element;
+		sGetLink(element)->previous = NULL;
+	}
+}
+
+
 // Swap
 DOUBLY_LINKED_LIST_TEMPLATE_LIST
 void
@@ -551,6 +567,7 @@ DOUBLY_LINKED_LIST_CLASS_NAME::Swap(Element* a, Element* b)
 	}
 }
 
+
 // MoveFrom
 DOUBLY_LINKED_LIST_TEMPLATE_LIST
 void
@@ -570,6 +587,7 @@ DOUBLY_LINKED_LIST_CLASS_NAME::MoveFrom(DOUBLY_LINKED_LIST_CLASS_NAME* fromList)
 	}
 }
 
+
 // RemoveAll
 DOUBLY_LINKED_LIST_TEMPLATE_LIST
 void
@@ -579,6 +597,7 @@ DOUBLY_LINKED_LIST_CLASS_NAME::RemoveAll()
 	fLast = NULL;
 }
 
+
 // RemoveHead
 DOUBLY_LINKED_LIST_TEMPLATE_LIST
 Element*