[IPX]: Use existing sock refcnt debugging infrastructure

Just like in the af_packet.c, the ipx_sock_nr variable is used
for debugging purposes.

Switch to using existing infrastructure. Thanks to Arnaldo for
pointing this out.

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>