udp: Prevent reuseport_select_sock from reading uninitialized socks

reuse->socks[] is modified concurrently by reuseport_add_sock. To
prevent reading values that have not been fully initialized, only read
the array up until the last known safe index instead of incorrectly
re-reading the last index of the array.

Fixes: acdcecc6 ("udp: correct reuseport selection with connected sockets")
Signed-off-by: Baptiste Lepers <baptiste.lepers@gmail.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20210107051110.12247-1-baptiste.lepers@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/sock_reuseport.c

@@ -293,7 +293,7 @@ struct sock *reuseport_select_sock(struct sock *sk,
 			i = j = reciprocal_scale(hash, socks);
 			while (reuse->socks[i]->sk_state == TCP_ESTABLISHED) {
 				i++;
-				if (i >= reuse->num_socks)
+				if (i >= socks)
 					i = 0;
 				if (i == j)
 					goto out;