net: mctp: Set SOCK_RCU_FREE

Bind lookup runs under RCU, so ensure that a socket doesn't go away in
the middle of a lookup.

Fixes: 833ef3b ("mctp: Populate socket implementation")
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Link: https://patch.msgid.link/20250410-mctp-rcu-sock-v1-1-872de9fdc877@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mctp/af_mctp.c

@@ -630,6 +630,9 @@ static int mctp_sk_hash(struct sock *sk)
 {
 	struct net *net = sock_net(sk);
 
+	/* Bind lookup runs under RCU, remain live during that. */
+	sock_set_flag(sk, SOCK_RCU_FREE);
+
 	mutex_lock(&net->mctp.bind_lock);
 	sk_add_node_rcu(sk, &net->mctp.binds);
 	mutex_unlock(&net->mctp.bind_lock);