ethtool: strset: report missing ETHTOOL_A_STRINGSET_ID via ext_ack

Strset needs ETHTOOL_A_STRINGSET_ID, use it as an example of
reporting attrs missing in nests.
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ethtool/strset.c

@@ -167,7 +167,7 @@ static int strset_get_id(const struct nlattr *nest, u32 *val,
 			       get_stringset_policy, extack);
 	if (ret < 0)
 		return ret;
-	if (!tb[ETHTOOL_A_STRINGSET_ID])
+	if (NL_REQ_ATTR_CHECK(extack, nest, tb, ETHTOOL_A_STRINGSET_ID))
 		return -EINVAL;
 
 	*val = nla_get_u32(tb[ETHTOOL_A_STRINGSET_ID]);