selftests: fib_tests: mute cleanup error message

In the end of the test, there will be an error message induced by the
`ip netns del ns1` command in cleanup()

  Tests passed: 201
  Tests failed:   0
  Cannot remove namespace file "/run/netns/ns1": No such file or directory

This can even be reproduced with just `./fib_tests.sh -h` as we're
calling cleanup() on exit.

Redirect the error message to /dev/null to mute it.

V2: Update commit message and fixes tag.
V3: resubmit due to missing netdev ML in V2

Fixes: b60417a9

 ("selftest: fib_tests: Always cleanup before exit")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>