pktgen: Automate flag enumeration for unknown flag handling

When specifying an unknown flag, it will print all available flags.
Currently, these flags are provided as fixed strings, which requires
manual updates when flags change. Replacing it with automated flag
enumeration.

Signed-off-by: Liang Chen <liangchen.linux@gmail.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Link: https://lore.kernel.org/r/20230920125658.46978-1-liangchen.linux@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>