af_unix: Define locking order for U_LOCK_SECOND in unix_state_double_lock().

unix_dgram_connect() and unix_dgram_{send,recv}msg() lock the socket
and peer in ascending order of the socket address.

Let's define the order as unix_state_lock_cmp_fn() instead of using
unix_state_lock_nested().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>