selftests/xsk: Add missing close() on netns fd

Commit 1034b03e ("selftests: xsk: Simplify cleanup of ifobjects")
removed close on netns fd, which is not correct, so let us restore it.

Fixes: 1034b03e ("selftests: xsk: Simplify cleanup of ifobjects")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220830133905.9945-1-maciej.fijalkowski@intel.com