net: dsa: use NET_NAME_PREDICTABLE for user ports with name given in DT

When a user port has a label in device tree, the corresponding
netdevice is, to quote include/uapi/linux/netdevice.h, "predictably
named by the kernel". This is also explicitly one of the intended use
cases for NET_NAME_PREDICTABLE, quoting 685343f ("net: add
name_assign_type netdev attribute"):

  NET_NAME_PREDICTABLE:
    The ifname has been assigned by the kernel in a predictable way
    [...] Examples include [...] and names deduced from hardware
    properties (including being given explicitly by the firmware).

Expose that information properly for the benefit of userspace tools
that make decisions based on the name_assign_type attribute,
e.g. a systemd-udev rule with "kernel" in NamePolicy.

Signed-off-by: Rasmus Villemoes <[email protected]>
Reviewed-by: Andrew Lunn <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

net/dsa/slave.c

@@ -2377,7 +2377,7 @@ int dsa_slave_create(struct dsa_port *port)
 
 	if (port->name) {
 		name = port->name;
-		assign_type = NET_NAME_UNKNOWN;
+		assign_type = NET_NAME_PREDICTABLE;
 	} else {
 		name = "eth%d";
 		assign_type = NET_NAME_UNKNOWN;