sock: don't enable netstamp for af_unix sockets

netstamp_needed is toggled for all socket families if they request
timestamping. But some protocols don't need the lower-layer timestamping
code at all. This patch starts disabling it for af-unix.

E.g. systemd enables timestamping during boot-up on the journald af-unix
sockets, thus causing the system to globally enable timestamping in the
lower networking stack. Still, it is very probable that timestamping
gets activated, by e.g. dhclient or various NTP implementations.

Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>