The test tool can check that the zerocopy number of completions value is
valid taking into consideration the number of datagram send calls. This can
catch the system into a state where the datagrams are still in the system
(for example in a qdisk, waiting for the network interface to return a
completion notification, etc).

This change adds a retry logic of computing the number of completions up to
a configurable (via CLI) timeout (default: 2 seconds).

Fixes: 79ebc3c2 ("net/udpgso_bench_tx: options to exercise TX CMSG")
Signed-off-by: Andrei Gherzan <andrei.gherzan@canonical.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20230201001612.515730-4-andrei.gherzan@canonical.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>