Linux PsList: Fix process list traversal

Fixes linux process listing by traversing the list backwards as well as
forwards while tracking seen process offsets to avoid duplicates.

volatility3/framework/plugins/linux/pslist.py

@@ -262,17 +262,27 @@ class PsList(interfaces.plugins.PluginInterface, timeliner.TimeLinerInterface):
         init_task = vmlinux.object_from_symbol(symbol_name="init_task")
 
         # Note that the init_task itself is not yielded, since "ps" also never shows it.
-        for task in init_task.tasks:
-            if not task.is_valid():
-                continue
+        seen = set()
+        for forward in (True, False):
+            for task in init_task.tasks.to_list(
+                symbol_type=init_task.vol.type_name,
+                member="tasks",
+                forward=forward,
+            ):
+                if task.vol.offset in seen:
+                    continue
+                seen.add(task.vol.offset)
 
-            if filter_func(task):
-                continue
+                if not task.is_valid():
+                    continue
 
-            yield task
+                if filter_func(task):
+                    continue
 
-            if include_threads:
-                yield from task.get_threads()
+                yield task
+
+                if include_threads:
+                    yield from task.get_threads()
 
     def run(self):
         pids = self.config.get("pid")