af_unix: Annotate data-race of sk->sk_state in unix_inq_len().

ioctl(SIOCINQ) calls unix_inq_len() that checks sk->sk_state first
and returns -EINVAL if it's TCP_LISTEN.

Then, for SOCK_STREAM sockets, unix_inq_len() returns the number of
bytes in recvq.

However, unix_inq_len() does not hold unix_state_lock(), and the
concurrent listen() might change the state after checking sk->sk_state.

If the race occurs, 0 is returned for the listener, instead of -EINVAL,
because the length of skb with embryo is 0.

We could hold unix_state_lock() in unix_inq_len(), but it's overkill
given the result is true for pre-listen() TCP_CLOSE state.

So, let's use READ_ONCE() for sk->sk_state in unix_inq_len().

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>