Comment 0 for bug 1908342

[Impact]
The cleanup function in the selftests/net/test_vxlan_under_vrf.sh script
that tries to delete hv-1 / hv-2 and vm-1 / vm-2 netns will generate some
unnecessary error messages:

Cannot remove namespace file "/run/netns/hv-2": No such file or directory
Cannot remove namespace file "/run/netns/vm-1": No such file or directory
Cannot remove namespace file "/run/netns/vm-2": No such file or directory

Redirect it to /dev/null like other commands in the cleanup function
to reduce confusion.

[Fix]
* 0e12c0271887f1 ("selftests: test_vxlan_under_vrf: mute unnecessary error message")

This patch can be cherry-picked into all affected series.

[Test]
Patch tested directly, the unnecessary error message won't be printed anymore

[Where problems could occur]
Change limited to test tools, it should not affect