selftests/net: Add SEQ number extension test

Check that on SEQ number wraparound there is no disruption or TCPAOBad
segments produced.

Sample of expected output:
> # ./seq-ext_ipv4
> 1..7
> # 1436[lib/setup.c:254] rand seed 1686611079
> TAP version 13
> ok 1 server alive
> ok 2 post-migrate connection alive
> ok 3 TCPAOGood counter increased 1002 => 3002
> ok 4 TCPAOGood counter increased 1003 => 3003
> ok 5 TCPAOBad counter didn't increase
> ok 6 TCPAOBad counter didn't increase
> ok 7 SEQ extension incremented: 1/1999, 1/998999
> # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>