rtnetlink: Handle IFLA_MASTER parameter when processing rtnl_newlink

Allow a master interface to be specified as one of the parameters when
creating a new interface via rtnl_newlink.  Previously this would
require invoking interface creation, waiting for it to complete, and
then separately binding that new interface to a master.

In particular, this is used when creating a macvlan child interface for
VRRP in a VRF configuration, allowing the interface creator to specify
directly what master interface should be inherited by the child,
without having to deal with asynchronous complications and potential
race conditions.

Signed-off-by: Theuns Verwoerd <theuns.verwoerd@alliedtelesis.co.nz>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2570,7 +2570,7 @@ replay:
 			return -ENODEV;
 		}
 
-		if (tb[IFLA_MAP] || tb[IFLA_MASTER] || tb[IFLA_PROTINFO])
+		if (tb[IFLA_MAP] || tb[IFLA_PROTINFO])
 			return -EOPNOTSUPP;
 
 		if (!ops) {
@@ -2652,6 +2652,11 @@ replay:
 			if (err < 0)
 				goto out_unregister;
 		}
+		if (tb[IFLA_MASTER]) {
+			err = do_set_master(dev, nla_get_u32(tb[IFLA_MASTER]));
+			if (err)
+				goto out_unregister;
+		}
 out:
 		if (link_net)
 			put_net(link_net);