ndisc: reject malformed captive portal URI with EBADMSG

This allows the correct, gracious, error handling to follow up in the
ndisc handler. Otherwise, an internal error is assumed and the interface
disabled.

Fixes: 9747955d2d ("ndisc: parse RFC8910 captive portal ipv6ra option")

src/network/networkd-ndisc.c

@@ -743,7 +743,7 @@ static int ndisc_router_process_captive_portal(Link *link, sd_ndisc_router *rt)
                 return r;
 
         if (!in_charset(captive_portal, URI_VALID))
-                return -EINVAL;
+                return -EBADMSG;
 
         if (!streq_ptr(link->ndisc_captive_portal, captive_portal)) {
                 free_and_replace(link->ndisc_captive_portal, captive_portal);