odhcpd: fix captive_portal_uri reset

odhcpd_reload() calls clean_interface() which does a memset() on the
given struct iface from iface->ra onwards, so captive_portal_uri and
captive_portal_uri_len have to be below iface->ra in order to be zeroed
on reload.

Otherwise, iface->captive_portal_uri will be free():d, but
iface->captive_portal_uri_len could still be > 0.

This is a bit of a footgun (one wouldn't normally expect the order of
struct members to matter), and should probably be dealt with in a more
thorough manner, but I've just done the minimum and added a warning in
src/odhcpd.h for now,

Signed-off-by: David Härdeman <david@hardeman.nu>
Link: openwrt#345
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

Author: Alphix

src/odhcpd.h

@@ -391,9 +391,7 @@ struct interface {
 	struct avl_tree dhcpv4_leases;
 	struct list_head dhcpv4_fr_ips;
 
-	// RFC8910
-	char *captive_portal_uri;
-	size_t captive_portal_uri_len;
+	/* NOTE: everything from this point is zeroed on odhcpd_reload() */
 
 	// Services
 	enum odhcpd_mode ra;
@@ -494,6 +492,10 @@ struct interface {
 	struct ra_pio *pios;
 	size_t pio_cnt;
 	bool pio_update;
+
+	// RFC8910
+	char *captive_portal_uri;
+	size_t captive_portal_uri_len;
 };
 
 extern struct avl_tree interfaces;