selftests/bpf: Implement sample udp/udp6 bpf_iter programs

On my VM, I got identical results between /proc/net/udp[6] and
the udp{4,6} bpf iterator.

For udp6:
  $ cat /sys/fs/bpf/p1
    sl  local_address                         remote_address                        st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops
   1405: 000080FE00000000FF7CC4D0D9EFE4FE:0222 00000000000000000000000000000000:0000 07 00000000:00000000 00:00000000 00000000   193        0 19183 2 0000000029eab111 0
  $ cat /proc/net/udp6
    sl  local_address                         remote_address                        st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops
   1405: 000080FE00000000FF7CC4D0D9EFE4FE:0222 00000000000000000000000000000000:0000 07 00000000:00000000 00:00000000 00000000   193        0 19183 2 0000000029eab111 0

For udp4:
  $ cat /sys/fs/bpf/p4
    sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops
   2007: 00000000:1F90 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 72540 2 000000004ede477a 0
  $ cat /proc/net/udp
    sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode ref pointer drops
   2007: 00000000:1F90 00000000:0000 07 00000000:00000000 00:00000000 00000000     0        0 72540 2 000000004ede477a 0

Signed-off-by: Yonghong Song <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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