networkd-ndisc: handle missing mtu gracefully (#4913)

At least bird's implementation of router advertisement does not
set MTU option by default (instead it supplies an option to the user).
In this case just leave MTU as it is.

src/network/networkd-ndisc.c

@@ -118,7 +118,9 @@ static void ndisc_router_process_default(Link *link, sd_ndisc_router *rt) {
         }
 
         r = sd_ndisc_router_get_mtu(rt, &mtu);
-        if (r < 0) {
+        if (r == -ENODATA)
+                mtu = 0;
+        else if (r < 0) {
                 log_link_warning_errno(link, r, "Failed to get default router MTU from RA: %m");
                 return;
         }