This is a follow-up to the commit 9b2b8633 ("bpf: Allow to use kfunc
XDP hints and frags together").

The are some possible implementations problems that may arise when providing
metadata specifically for multi-buffer packets, therefore there must be a
possibility to test such option separately.

Add an option to use multi-buffer AF_XDP xdp_hw_metadata and mark used XDP
program as capable to use frags.

As for now, xdp_hw_metadata accepts no options, so add simple option
parsing logic and a help message.

For quick reference, also add an ingress packet generation command to the
help message. The command comes from [0].

Example of output for multi-buffer packet:

  xsk_ring_cons__peek: 1
  0xead018: rx_desc[15]->addr=10000000000f000 addr=f100 comp_addr=f000
  rx_hash: 0x5789FCBB with RSS type:0x29
  rx_timestamp:  1696856851535324697 (sec:1696856851.5353)
  XDP RX-time:   1696856843158256391 (sec:1696856843.1583)
  	delta sec:-8.3771 (-8377068.306 usec)
  AF_XDP time:   1696856843158413078 (sec:1696856843.1584)
  	delta sec:0.0002 (156.687 usec)
  0xead018: complete idx=23 addr=f000
  xsk_ring_cons__peek: 1
  0xead018: rx_desc[16]->addr=100000000008000 addr=8100 comp_addr=8000
  0xead018: complete idx=24 addr=8000
  xsk_ring_cons__peek: 1
  0xead018: rx_desc[17]->addr=100000000009000 addr=9100 comp_addr=9000 EoP
  0xead018: complete idx=25 addr=9000

Metadata is printed for the first packet only.

  [0] https://lore.kernel.org/all/20230119221536.3349901-18-sdf@google.com/Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20231017162800.24080-1-larysa.zaremba@intel.com