selftests: net: rps_default_mask.sh: delete veth link specifically

When deleting the netns and recreating a new one while re-adding the
veth interface, there is a small window of time during which the old
veth interface has not yet been removed. This can cause the new addition
to fail. To resolve this issue, we can either wait for a short while to
ensure that the old veth interface is deleted, or we can specifically
remove the veth interface.

Before this patch:
  # ./rps_default_mask.sh
  empty rps_default_mask                                      [ ok ]
  changing rps_default_mask dont affect existing devices      [ ok ]
  changing rps_default_mask dont affect existing netns        [ ok ]
  changing rps_default_mask affect newly created devices      [ ok ]
  changing rps_default_mask don't affect newly child netns[II][ ok ]
  rps_default_mask is 0 by default in child netns             [ ok ]
  RTNETLINK answers: File exists
  changing rps_default_mask in child ns don't affect the main one[ ok ]
  cat: /sys/class/net/vethC11an1/queues/rx-0/rps_cpus: No such file or directory
  changing rps_default_mask in child ns affects new childns devices./rps_default_mask.sh: line 36: [: -eq: unary operator expected
  [fail] expected 1 found
  changing rps_default_mask in child ns don't affect existing devices[ ok ]

After this patch:
  # ./rps_default_mask.sh
  empty rps_default_mask                                      [ ok ]
  changing rps_default_mask dont affect existing devices      [ ok ]
  changing rps_default_mask dont affect existing netns        [ ok ]
  changing rps_default_mask affect newly created devices      [ ok ]
  changing rps_default_mask don't affect newly child netns[II][ ok ]
  rps_default_mask is 0 by default in child netns             [ ok ]
  changing rps_default_mask in child ns don't affect the main one[ ok ]
  changing rps_default_mask in child ns affects new childns devices[ ok ]
  changing rps_default_mask in child ns don't affect existing devices[ ok ]

Fixes: 3a7d84eae0 ("self-tests: more rps self tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20230404072411.879476-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/rps_default_mask.sh

@@ -60,6 +60,7 @@ ip link set dev $VETH up
 ip -n $NETNS link set dev $VETH up
 chk_rps "changing rps_default_mask affect newly created devices" "" $VETH 3
 chk_rps "changing rps_default_mask don't affect newly child netns[II]" $NETNS $VETH 0
+ip link del dev $VETH
 ip netns del $NETNS
 
 setup