selftests: xsk: cleanup veth pair at ctrl-c

Remove the veth pair when the tests are aborted by pressing
ctrl-c. Currently in this situation, the veth pair is left on the
system polluting the netdev space.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20220510115604.8717-7-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>