unix_poll() and unix_dgram_poll() read sk->sk_state locklessly and
calls unix_writable() which also reads sk->sk_state without holding
unix_state_lock().

Let's use READ_ONCE() in unix_poll() and unix_dgram_poll() and pass
it to unix_writable().

While at it, we remove TCP_SYN_SENT check in unix_dgram_poll() as
that state does not exist for AF_UNIX socket since the code was added.

Fixes: 1586a587 ("af_unix: do not report POLLOUT on listeners")
Fixes: 3c73419c ("af_unix: fix 'poll for write'/ connected DGRAM sockets")
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>