mptcp: init sk->sk_prot in build_msk()

The 'sk_prot' field in token KUNIT self-tests will be dereferenced in
mptcp_token_new_connect(). Therefore, init it with tcp_prot.
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/token_test.c

@@ -57,6 +57,9 @@ static struct mptcp_sock *build_msk(struct kunit *test)
 	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, msk);
 	refcount_set(&((struct sock *)msk)->sk_refcnt, 1);
 	sock_net_set((struct sock *)msk, &init_net);
+
+	/* be sure the token helpers can dereference sk->sk_prot */
+	((struct sock *)msk)->sk_prot = &tcp_prot;
 	return msk;
 }