config: add default value for dhcpv6_pd_min_len

Right now, dhcpv6_pd_min_len is zero, meaning any request would be
accepted (as long as it fits what we have available).

That means that with a /56 (a common residential scenario), a downstream
router can request a /57 and gobble up 1/2 of the available address
space. Even a default dhcpv6_pd_min_len value of 60 means that we'd
support 15 PDs in total.

So let's err on the side of caution and set /62 as the default, people
who want larger allocations can easily override the default.

(cherry picked from commit 35f0e05)
Signed-off-by: David Härdeman <david@hardeman.nu>
Link: openwrt#357
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

Author: Alphix

README.md

@@ -89,7 +89,7 @@ and may also receive information from ubus
 | dhcpv6_na		|bool	| 1	| DHCPv6 stateful addressing hands out IA_NA - Internet Address - Network Address |
 | dhcpv6_pd		|bool	| 1	| DHCPv6 stateful addressing hands out IA_PD - Internet Address - Prefix Delegation (PD) |
 | dhcpv6_pd_preferred   |bool | 0 | Set the DHCPv6-PD Preferred (P) flag in outgoing ICMPv6 RA message PIOs (RFC9762); requires `dhcpv6` and `dhcpv6_pd`. |
-| dhcpv6_pd_min_len	|integer| -	| Minimum prefix length to delegate with IA_PD (value is adjusted if needed to be greater than the interface prefix length).  Range [1,64] |
+| dhcpv6_pd_min_len	|integer| 62	| Minimum prefix length to delegate with IA_PD (adjusted, if necessary, to be longer than the interface prefix length).  Range [1,64] |
 | router		|list	|`<local address>`| IPv4 addresses of routers on a given subnet (provided via DHCPv4, should be in order of preference) |
 | dns			|list	|`<local address>`| DNS servers to announce, accepts IPv4 and IPv6 |
 | dnr			|list	|disabled| Encrypted DNS servers to announce, `<priority> <domain name> [<comma separated IP addresses> <SvcParams (key=value)>...]` |

src/config.c

@@ -69,7 +69,8 @@ struct sys_conf sys_conf = {
 #define HOSTID_LEN_MAX	64
 #define HOSTID_LEN_DEFAULT HOSTID_LEN_MIN
 
-#define PD_MIN_LEN_MAX	64
+#define PD_MIN_LEN_MAX		64
+#define PD_MIN_LEN_DEFAULT	62
 
 #define OAF_DHCPV6	(OAF_DHCPV6_NA | OAF_DHCPV6_PD)
 
@@ -324,7 +325,7 @@ static void set_interface_defaults(struct interface *iface)
 	iface->dhcpv6_assignall = true;
 	iface->dhcpv6_pd = true;
 	iface->dhcpv6_pd_preferred = false;
-	iface->dhcpv6_pd_min_len = 0;
+	iface->dhcpv6_pd_min_len = PD_MIN_LEN_DEFAULT;
 	iface->dhcpv6_na = true;
 	iface->dhcpv6_hostid_len = HOSTID_LEN_DEFAULT;
 	iface->dns_service = true;