ndp: Set (ICMP) hop limit to 255 in router advertisement

Found while re-reading this part, zero works as well, but a
host might legitimately refuse a value that's below a given
threshold.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

ndp.c

@@ -100,6 +100,7 @@ int ndp(struct ctx *c, struct ethhdr *eh, size_t len)
 		info("NDP: received RS, sending RA");
 		ihr->icmp6_type = RA;
 		ihr->icmp6_code = 0;
+		ihr->icmp6_hop_limit = 255;
 		ihr->icmp6_rt_lifetime = htons(9000);
 		ihr->icmp6_addrconf_managed = 1;