l2tpv3: fix cookie decoding

If a 32 bits l2tpv3 frame cookie MSB if set to 1, the cast to uint64_t
cookie will spread 1 to the four most significant bytes.
Then the condition (cookie != s->rx_cookie) becomes false.
Signed-off-by: NAlexis Dambricourt <alexis.dambricourt@gmail.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>

net/l2tpv3.c

@@ -325,7 +325,7 @@ static int l2tpv3_verify_header(NetL2TPV3State *s, uint8_t *buf)
         if (s->cookie_is_64) {
             cookie = ldq_be_p(buf + s->cookie_offset);
         } else {
-            cookie = ldl_be_p(buf + s->cookie_offset);
+            cookie = ldl_be_p(buf + s->cookie_offset) & 0xffffffffULL;
         }
         if (cookie != s->rx_cookie) {
             if (!s->header_mismatch) {