kernel/vm: Adjust next-insert hint on area removal.

This doesn't matter so much because the hint's used for
an AVL tree lookup both for next and previous, but it
probably does at least a bit because the new area's size
is used in the tree lookups.

src/system/kernel/vm/VMUserAddressSpace.cpp

@@ -218,6 +218,9 @@ VMUserAddressSpace::RemoveArea(VMArea* _area, uint32 allocationFlags)
 		IncrementChangeCount();
 		fFreeSpace += area->Size();
 	}
+
+	if ((area->Base() + area->Size()) == fNextInsertHint)
+		fNextInsertHint -= area->Size();
 }