mptcp: token kunit: set protocol

As it would be done when initiating an MPTCP sock.

This is not strictly needed for this test, but it will be when a later
patch will check if the right protocol is being used when calling
mptcp_sk().
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-next-20240223-misc-improvements-v1-2-b6c8a10396bd@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/token_test.c

@@ -52,14 +52,19 @@ static struct mptcp_subflow_context *build_ctx(struct kunit *test)
 static struct mptcp_sock *build_msk(struct kunit *test)
 {
 	struct mptcp_sock *msk;
+	struct sock *sk;
 
 	msk = kunit_kzalloc(test, sizeof(struct mptcp_sock), GFP_USER);
 	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, msk);
 	refcount_set(&((struct sock *)msk)->sk_refcnt, 1);
 	sock_net_set((struct sock *)msk, &init_net);
 
+	sk = (struct sock *)msk;
+
 	/* be sure the token helpers can dereference sk->sk_prot */
-	((struct sock *)msk)->sk_prot = &tcp_prot;
+	sk->sk_prot = &tcp_prot;
+	sk->sk_protocol = IPPROTO_MPTCP;
+
 	return msk;
 }