net/key/af_key.c: add range checks on ->sadb_x_policy_len

Because sizeof() is size_t then if "len" is negative, it counts as a
large positive value.

The call tree looks like:
pfkey_sendmsg()
-> pfkey_process()
   -> pfkey_spdadd()
      -> parse_ipsecrequests()
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/key/af_key.c

@@ -1923,6 +1923,9 @@ parse_ipsecrequests(struct xfrm_policy *xp, struct sadb_x_policy *pol)
 	int len = pol->sadb_x_policy_len*8 - sizeof(struct sadb_x_policy);
 	struct sadb_x_ipsecrequest *rq = (void*)(pol+1);
 
+	if (pol->sadb_x_policy_len * 8 < sizeof(struct sadb_x_policy))
+		return -EINVAL;
+
 	while (len >= sizeof(struct sadb_x_ipsecrequest)) {
 		if ((err = parse_ipsecrequest(xp, rq)) < 0)
 			return err;