selftest: bpf: Test bpf_sk_assign_tcp_reqsk().

This commit adds a sample selftest to demonstrate how we can use
bpf_sk_assign_tcp_reqsk() as the backend of SYN Proxy.

The test creates IPv4/IPv6 x TCP connections and transfer messages
over them on lo with BPF tc prog attached.

The tc prog will process SYN and returns SYN+ACK with the following
ISN and TS.  In a real use case, this part will be done by other
hosts.

        MSB                                   LSB
  ISN:  | 31 ... 8 | 7 6 |   5 |    4 | 3 2 1 0 |
        |   Hash_1 | MSS | ECN | SACK |  WScale |

  TS:   | 31 ... 8 |          7 ... 0           |
        |   Random |           Hash_2           |

  WScale in SYN is reused in SYN+ACK.

The client returns ACK, and tc prog will recalculate ISN and TS
from ACK and validate SYN Cookie.

If it's valid, the prog calls kfunc to allocate a reqsk for skb and
configure the reqsk based on the argument created from SYN Cookie.

Later, the reqsk will be processed in cookie_v[46]_check() to create
a connection.

Signed-off-by: Kuniyuki Iwashima <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Martin KaFai Lau <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>

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