selftests/bpf: Add tests for reading a dangling map iter fd

selftests/bpf: Add tests for reading a dangling map iter fd

After closing both related link fd and map fd, reading the map
iterator fd to ensure it is OK to do so.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220810080538.1845898-8-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>