af_unix: Set sk_peer_pid/sk_peer_cred locklessly for new socket.

init_peercred() is called in 3 places:

  1. socketpair() : both sockets
  2. connect()    : child socket
  3. listen()     : listening socket

The first two need not hold sk_peer_lock because no one can
touch the socket.

Let's set cred/pid without holding lock for the two cases and
rename the old init_peercred() to update_peercred() to properly
reflect the use case.

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