selftests: net: test extacks in netlink dumps

Test that extacks in dumps work. The test fills up the receive buffer
to test both the inline dump (as part of sendmsg()) and delayed one
(run during recvmsg()).

Use YNL helpers to parse the messages. We need to add the test to YNL
file to make sure the right include path are used.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20241119224432.1713040-2-kuba@kernel.org


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