af_unix: Don't acquire unix_state_lock() for sock_i_ino().

sk_diag_dump_peer() and sk_diag_dump() call unix_state_lock() for
sock_i_ino() which reads SOCK_INODE(sk->sk_socket)->i_ino, but it's
protected by sk->sk_callback_lock.

Let's remove unnecessary unix_state_lock().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>