platform: don't update IPv6 temporary addresses

The kernel already takes care of adding and updating temporary
addresses when an address with IFA_F_MANAGETEMPADDR flag is added or
updated; doing it also in nm_platform_ip6_address_sync() can overwrite
the changes done by kernel, especially because since commit
0a0bca9c7f ("ip6-config: sort addresses only when reading the
property value") there is no guarantee that temporary addresses are
before the public ones in the IPv6 configuration.

Still delete temporary addresses, but don't add or update them.

(cherry picked from commit 1dbd9d7948)

src/platform/nm-platform.c

@@ -2815,6 +2815,11 @@ nm_platform_ip6_address_sync (NMPlatform *self, int ifindex, const GArray *known
 		const NMPlatformIP6Address *known_address = &g_array_index (known_addresses, NMPlatformIP6Address, i);
 		guint32 lifetime, preferred;
 
+		if (NM_FLAGS_HAS (known_address->n_ifa_flags, IFA_F_TEMPORARY)) {
+			/* Kernel manages these */
+			continue;
+		}
+
 		if (!nm_utils_lifetime_get (known_address->timestamp, known_address->lifetime, known_address->preferred,
 		                            now, &lifetime, &preferred))
 			continue;