Clean up intfs in root NS, and avoid deleting HW intfs

It appears that under certain conditions, such as when a
namespace exits, both ends of a veth pair may get dumped
into the root namespace. We therefore now remove an interface
both from its home namespace and from the root namespace.

mininet/link.py

@@ -25,7 +25,7 @@ Link: basic link class for creating veth pairs
 """
 
 from mininet.log import info, error, debug
-from mininet.util import makeIntfPair
+from mininet.util import makeIntfPair, quietRun
 from time import sleep
 import re
 
@@ -162,8 +162,9 @@ class Intf( object ):
     def delete( self ):
         "Delete interface"
         self.cmd( 'ip link del ' + self.name )
-        # Does it help to sleep to let things run?
-        sleep( 0.001 )
+        if self.node.inNamespace:
+            # Link may have been dumped into root NS
+            quietRun( 'ip link del ' + self.name )
 
     def __repr__( self ):
         return '<%s %s>' % ( self.__class__.__name__, self.name )