udev: introduce NAMING_STABLE_VIRTUAL_MACS (retroactively)

This is for 6d36464065. It turns out that this is causing more problems than
expected. Let's retroactively introduce naming scheme v241 to conditionalize
this change.

Follow-up for #12792 and 6d36464065. See also
https://bugzilla.suse.com/show_bug.cgi?id=1136600.

$ SYSTEMD_LOG_LEVEL=debug NET_NAMING_SCHEME=v240 build/udevadm test-builtin net_setup_link /sys/class/net/br11
$ SYSTEMD_LOG_LEVEL=debug NET_NAMING_SCHEME=v241 build/udevadm test-builtin net_setup_link /sys/class/net/br11
...
@@ -20,11 +20,13 @@
 link_config: could not set ethtool features for br11
 Could not set offload features of br11: Operation not permitted
 br11: Device has name_assign_type=3
-Using interface naming scheme 'v240'.
+Using interface naming scheme 'v241'.
 br11: Policy *keep*: keeping existing userspace name
 br11: Device has addr_assign_type=1
-br11: No stable identifying information found
-br11: Could not generate persistent MAC: No data available
+br11: Using "br11" as stable identifying information
+br11: Using generated persistent MAC address
+Could not set Alias=, MACAddress= or MTU= on br11: Operation not permitted
+br11: Could not apply link config, ignoring: Operation not permitted
 Unload module index
 Unloaded link configuration context.
 ID_NET_DRIVER=bridge

src/libsystemd-network/network-internal.c

@@ -39,7 +39,7 @@ const char *net_get_name_persistent(sd_device *device) {
 
 #define HASH_KEY SD_ID128_MAKE(d3,1e,48,fa,90,fe,4b,4c,9d,af,d5,d7,a1,b1,2e,8a)
 
-int net_get_unique_predictable_data(sd_device *device, uint64_t *result) {
+int net_get_unique_predictable_data(sd_device *device, bool use_sysname, uint64_t *result) {
         size_t l, sz = 0;
         const char *name;
         int r;
@@ -50,7 +50,7 @@ int net_get_unique_predictable_data(sd_device *device, uint64_t *result) {
         /* net_get_name_persistent() will return one of the device names based on stable information about
          * the device. If this is not available, we fall back to using the actual device name. */
         name = net_get_name_persistent(device);
-        if (!name)
+        if (!name && use_sysname)
                 (void) sd_device_get_sysname(device, &name);
         if (!name)
                 return log_device_debug_errno(device, SYNTHETIC_ERRNO(ENODATA),