[PACKET]: Use existing sock refcnt debugging infrastructure

The packet_socks_nr variable is used purely for debugging
the number of sockets.

As Arnaldo pointed out, there's already an infrastructure
for this purposes, so switch to using it.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>