udev,networkd: use the interface name as fallback basis for MAC and IPv4LL seed

udev,networkd: use the interface name as fallback basis for MAC and IPv4LL seed

Fixes #3374. The problem is that we set MACPolicy=persistent (i.e. we would
like to generate persistent MAC addresses for interfaces which don't have a
fixed MAC address), but various virtual interfaces including bridges, tun/tap,
bonds, etc., do not not have the necessary ID_NET_NAME_* attributes and udev
would not assing the address and warn:
  Could not generate persistent MAC address for $name: No such file or directory

Basic requirements which I think a solution for this needs to satisfy:

1. No changes to MAC address generation for those cases which are currently
  handled successfully. This means that net_get_unique_predictable_data() must
  keep returning the same answer, which in turn means net_get_name() must keep
  returning the same answer. We can only add more things we look at with lower
  priority so that we start to cover cases which were not covered before.

2. Like 1, but for IPvLL seed and DHCP IAD. This is less important, but "nice
  to have".

3. Keep MACPolicy=persistent. If people don't want it, they can always apply
  local configuration, but in general stable MACs are a good thing. I have never
  seen anyone complain about that.

== Various approaches that have been proposed

=== https://github.com/systemd/systemd/issues/3374#issuecomment-223753264 (tomty89)
if !ID_BUS and INTERFACE, use INTERFACE

I think this almost does the good thing, but I don't see the reason to reject ID_BUS
(i.e. physical hardware). Stable MACs are very useful for physical hardware that has
no physical MAC.

=== https://github.com/systemd/systemd/issues/3374#issuecomment-224733069 (teg)
if (should_rename(device, true))

This means looking at name_assign_type. In particular for
NET_NAME_USER should_rename(..., true) returns true. It only returns false
for NET_NAME_PREDICTABLE. So this would cover stuff like br0, bond0, etc,
but would not cover lo and other devices with predictable names. That doesn't
make much sense.

But did teg mean should_rename() or !should_rename()?

=== https://github.com/systemd/systemd/issues/3374#issuecomment-234628502 (tomty89):
+ if (!should_rename(device, true))
+        return udev_device_get_sysname(device)

This covers only devices with NET_NAME_PREDICTABLE. Since the problem applies as
much to bridges and such, this isn't neough.

=== https://github.com/systemd/systemd/issues/3374#issuecomment-281745967  (grafi-tt)
+        /* if the machine doesn't provide data about the device, use the ifname specified by userspace
+        * (this is the case when the device is virtual, e.g., bridge or bond) */
+        s = udev_device_get_sysattr_value(device, "name_assign_type");
+        if (s && safe_atou(s, &type) >= 0 && type == NET_NAME_USER)
+                return udev_device_get_sysname(device);

This does not cover bond0, vnet0, tun/tap and similar.
grafi-tt also proposes patching the kernel, but *not* setting name_assign_type
seems intentional in those cases, because the device name is a result of
enumeration, not set by the userspace.

=== https://github.com/systemd/systemd/issues/3374#issuecomment-288882355 (tomty89)
(also PR #11372)
- MACAddressPolicy=persistent

This break requirement 3. above. It would solve the immediate problem, but I
think the disruption is too big.

=== This patch

This patch means that we will set a "stable" MAC for pretty much any virtual
device by default, where "stable" means keyed off the machine-id and interface
name.

It seems like a big change, but we already did this for most physical devices.
Doing it also for virtual devices doesn't seem like a big issue. It will make
the setup and monitoring of virtualized networks slightly nicer. I don't think
anyone is depending on having the MAC address changed when those devices are
destoryed and recreated. If they do, they'd have to change MACAddressPolicy=.

== Implementation
net_get_name() is called from dhcp_ident_set_iaid() so I didn't change
net_get_name() like in grafi-tt's patch, but net_get_unique_predictable_data().

net_get_unique_predictable_data() is called from get_mac() in link-config.c
and sd_ipv4ll_set_address_seed(), so both of those code paths are affected
and will now get data in some cases where they errored out previously.

The return code is changed to -ENODATA since that gives a nicer error string.