selftests/net: make so_txtime more robust to timer variance

The SO_TXTIME test depends on accurate timers. In some virtualized
environments the test has been reported to be flaky. This is easily
reproduced by disabling kvm acceleration in Qemu.

Allow greater variance in a run and retry to further reduce flakiness.

Observed errors are one of two kinds: either the packet arrives too
early or late at recv(), or it was dropped in the qdisc itself and the
recv() call times out.

In the latter case, the qdisc queues a notification to the error
queue of the send socket. Also explicitly report this cause.

Link: https://lore.kernel.org/netdev/CA+FuTSdYOnJCsGuj43xwV1jxvYsaoa_LzHQF9qMyhrkLrivxKw@mail.gmail.com


Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>