networkd: Allow ndisc client and radv to configure on later stage

The conf
```
[Match]
Name=enp7s0f0

[Network]
DHCP=true
IPv6PrivacyExtensions=true
IPv6AcceptRA=true <============================================
IPv6MTUBytes=1492
DNSDefaultRoute=false

[DHCP]
UseDNS=false
UseHostname=false
UseNTP=false
UseDomains=route
UseMTU=true

[IPv6AcceptRA]
UseDNS=no
UseDomains=route
```

The log
```
enp7s0f0: Gained IPv6LL
Assertion 'link->ndisc' failed at ../systemd-stable/src/network/networkd-link.c:1829, function link_acquire_ipv6_conf(). Abor
Assertion 'link->radv' failed at ../systemd/src/network/networkd-link.c:1842, function link_acquire_ipv6_conf(). Aborting.
Aborted
```

From the conf it's clear that RA accepting is enables. But hmm
initially when networkd started the ipv6ll was not there. Later
on it gained that and tried to start ndisc client and radv .
Hence crashed. So allow ndisc client to configure on later stage.

Closes systemd#12452

src/network/networkd-link.c

@@ -1825,7 +1825,11 @@ static int link_acquire_ipv6_conf(Link *link) {
         assert(link);
 
         if (link_ipv6_accept_ra_enabled(link)) {
-                assert(link->ndisc);
+                if (!link->ndisc) {
+                        r = ndisc_configure(link);
+                        if (r < 0)
+                                return r;
+                }
 
                 log_link_debug(link, "Discovering IPv6 routers");
 
@@ -1835,9 +1839,14 @@ static int link_acquire_ipv6_conf(Link *link) {
         }
 
         if (link_radv_enabled(link)) {
-                assert(link->radv);
                 assert(in_addr_is_link_local(AF_INET6, (const union in_addr_union*)&link->ipv6ll_address) > 0);
 
+                if (!link->radv) {
+                        r = radv_configure(link);
+                        if (r < 0)
+                                return r;
+                }
+
                 log_link_debug(link, "Starting IPv6 Router Advertisements");
 
                 r = sd_radv_start(link->radv);