ipv6: tcp: fix panic in SYN processing

commit 72a3effa ([NET]: Size listen hash tables using backlog
hint) added a bug allowing inet6_synq_hash() to return an out of bound
array index, because of u16 overflow.

Bug can happen if system admins set net.core.somaxconn &
net.ipv4.tcp_max_syn_backlog sysctls to values greater than 65536
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/inet6_connection_sock.c

@@ -85,7 +85,7 @@ struct dst_entry *inet6_csk_route_req(struct sock *sk,
  * request_sock (formerly open request) hash tables.
  */
 static u32 inet6_synq_hash(const struct in6_addr *raddr, const __be16 rport,
-			   const u32 rnd, const u16 synq_hsize)
+			   const u32 rnd, const u32 synq_hsize)
 {
 	u32 c;