tc: fix get_qdiscs(index=...)

The old version could filter qdiscs by the interface index, fix that
in the new API.

Bug-Url: #1225

pyroute2/iproute/linux.py

@@ -416,7 +416,7 @@ async def get_qdiscs(self, index=None):
 
         A compatibility method, == .tc("dump")
         '''
-        return await self.tc('dump')
+        return await self.tc('dump', index=index)
 
     async def get_filters(self, index=0, handle=0, parent=0):
         '''