ipv6/addrconf: only check invalid header values when NETLINK_F_STRICT_CHK is set

In commit 4b1373de ("net: ipv6: addr: perform strict checks also for
doit handlers") we add strict check for inet6_rtm_getaddr(). But we did
the invalid header values check before checking if NETLINK_F_STRICT_CHK
is set. This may break backwards compatibility if user already set the
ifm->ifa_prefixlen, ifm->ifa_flags, ifm->ifa_scope in their netlink code.

I didn't move the nlmsg_len check because I thought it's a valid check.
Reported-by: Jianlin Shi <jishi@redhat.com>
Fixes: 4b1373de ("net: ipv6: addr: perform strict checks also for doit handlers")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

net/ipv6/addrconf.c

@@ -5231,16 +5231,16 @@ static int inet6_rtm_valid_getaddr_req(struct sk_buff *skb,
 		return -EINVAL;
 	}
 
+	if (!netlink_strict_get_check(skb))
+		return nlmsg_parse_deprecated(nlh, sizeof(*ifm), tb, IFA_MAX,
+					      ifa_ipv6_policy, extack);
+
 	ifm = nlmsg_data(nlh);
 	if (ifm->ifa_prefixlen || ifm->ifa_flags || ifm->ifa_scope) {
 		NL_SET_ERR_MSG_MOD(extack, "Invalid values in header for get address request");
 		return -EINVAL;
 	}
 
-	if (!netlink_strict_get_check(skb))
-		return nlmsg_parse_deprecated(nlh, sizeof(*ifm), tb, IFA_MAX,
-					      ifa_ipv6_policy, extack);
-
 	err = nlmsg_parse_deprecated_strict(nlh, sizeof(*ifm), tb, IFA_MAX,
 					    ifa_ipv6_policy, extack);
 	if (err)