selftests/bpf: Simple program to dump XDP RX metadata

To be used for verification of driver implementations. Note that
the skb path is gone from the series, but I'm still keeping the
implementation for any possible future work.

$ xdp_hw_metadata <ifname>

On the other machine:

$ echo -n xdp | nc -u -q1 <target> 9091 # for AF_XDP
$ echo -n skb | nc -u -q1 <target> 9092 # for skb

Sample output:

  # xdp
  xsk_ring_cons__peek: 1
  0x19f9090: rx_desc[0]->addr=100000000008000 addr=8100 comp_addr=8000
  rx_timestamp_supported: 1
  rx_timestamp: 1667850075063948829
  0x19f9090: complete idx=8 addr=8000

  # skb
  found skb hwtstamp = 1668314052.854274681

Decoding:
  # xdp
  rx_timestamp=1667850075.063948829

  $ date -d @1667850075
  Mon Nov  7 11:41:15 AM PST 2022
  $ date
  Mon Nov  7 11:42:05 AM PST 2022

  # skb
  $ date -d @1668314052
  Sat Nov 12 08:34:12 PM PST 2022
  $ date
  Sat Nov 12 08:37:06 PM PST 2022

Cc: John Fastabend <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Martin KaFai Lau <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Willem de Bruijn <[email protected]>
Cc: Jesper Dangaard Brouer <[email protected]>
Cc: Anatoly Burakov <[email protected]>
Cc: Alexander Lobakin <[email protected]>
Cc: Magnus Karlsson <[email protected]>
Cc: Maryam Tahhan <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Stanislav Fomichev <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>

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