tcp: Fix data-races around sysctl_tcp_migrate_req.

While reading sysctl_tcp_migrate_req, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its readers.

Fixes: f9ac779f881c ("net: Introduce net.ipv4.tcp_migrate_req.")
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/core/sock_reuseport.c

···
 		prog = rcu_dereference_protected(reuse->prog,
 						 lockdep_is_held(&reuseport_lock));
 
-		if (sock_net(sk)->ipv4.sysctl_tcp_migrate_req ||
+		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_migrate_req) ||
 		    (prog && prog->expected_attach_type == BPF_SK_REUSEPORT_SELECT_OR_MIGRATE)) {
 			/* Migration capable, move sk from the listening section
 			 * to the closed section.
···
 	hash = migrating_sk->sk_hash;
 	prog = rcu_dereference(reuse->prog);
 	if (!prog || prog->expected_attach_type != BPF_SK_REUSEPORT_SELECT_OR_MIGRATE) {
-		if (sock_net(sk)->ipv4.sysctl_tcp_migrate_req)
+		if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_migrate_req))
 			goto select_by_hash;
 		goto failure;
 	}