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 <[e-mail]>
Signed-off-by: Matthieu Baerts (NGI0) <[e-mail]>
Link: https://lore.kernel.org/r/2024022[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/mptcp/token_test.c

···
 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;
 }