Set interface with longest prefix in DHCP & DHCPv6 lease

- With nested prefixes reside on different interfaces of single host

  (e.g., in 6to4, 2002::/16 on WAN and 2002:<IPv4>:<subnet>::/64 on LAN),

  current matching mechanism might return the interface with shorter prefix

  length instead of the longer one, if it appears later in the netlink message.

Signed-off-by: Lung-Pin Chang <changlp@cs.nctu.edu.tw>

src/util.c

@@ -319,6 +319,18 @@ time_t dnsmasq_time(void)
 #endif
 }
 
+int netmask_length(struct in_addr mask)
+{
+  int zero_count = 0;
+
+  while (0x0 == (mask.s_addr & 0x1)) {
+    mask.s_addr >>= 1;
+    ++zero_count;
+  }
+
+  return 32 - zero_count;
+}
+
 int is_same_net(struct in_addr a, struct in_addr b, struct in_addr mask)
 {
   return (a.s_addr & mask.s_addr) == (b.s_addr & mask.s_addr);