rtnetlink: Add more extack messages to rtnl_newlink

Add extack arg to the nla_parse_nested calls in rtnl_newlink, and
add messages for unknown device type and link network namespace id.
In particular, it improves the failure message when the wrong link
type is used. From
    $ ip li add bond1 type bonding
    RTNETLINK answers: Operation not supported
to
    $ ip li add bond1 type bonding
    Error: Unknown device type.

(The module name is bonding but the link type is bond.)
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -3054,7 +3054,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			if (ops->maxtype && linkinfo[IFLA_INFO_DATA]) {
 				err = nla_parse_nested(attr, ops->maxtype,
 						       linkinfo[IFLA_INFO_DATA],
-						       ops->policy, NULL);
+						       ops->policy, extack);
 				if (err < 0)
 					return err;
 				data = attr;
@@ -3076,7 +3076,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 						       m_ops->slave_maxtype,
 						       linkinfo[IFLA_INFO_SLAVE_DATA],
 						       m_ops->slave_policy,
-						       NULL);
+						       extack);
 				if (err < 0)
 					return err;
 				slave_data = slave_attr;
@@ -3140,6 +3140,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 					goto replay;
 			}
 #endif
+			NL_SET_ERR_MSG(extack, "Unknown device type");
 			return -EOPNOTSUPP;
 		}
 
@@ -3160,6 +3161,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 
 			link_net = get_net_ns_by_id(dest_net, id);
 			if (!link_net) {
+				NL_SET_ERR_MSG(extack, "Unknown network namespace id");
 				err =  -EINVAL;
 				goto out;
 			}