selftests: net: test SO_PRIORITY ancillary data with cmsg_sender

Extend cmsg_sender.c with a new option '-Q' to send SO_PRIORITY
ancillary data.

cmsg_so_priority.sh script added to validate SO_PRIORITY behavior
by creating VLAN device with egress QoS mapping and testing packet
priorities using flower filters. Verify that packets with different
priorities are correctly matched and counted by filters for multiple
protocols and IP versions.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Suggested-by: Ido Schimmel <idosch@idosch.org>
Signed-off-by: Anna Emese Nyiri <annaemesenyiri@gmail.com>
Link: https://patch.msgid.link/20241213084457.45120-4-annaemesenyiri@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>