net: af_unix can make unix_nr_socks visbile in /proc

Currently, /proc/net/protocols displays socket counts only for TCP/TCPv6
protocols

We can provide unix_nr_socks for free here, this counter being
already maintained in af_unix

Before patch :

# grep UNIX /proc/net/protocols
UNIX       428     -1      -1   NI       0   yes  kernel

After patch :

# grep UNIX /proc/net/protocols
UNIX       428     98      -1   NI       0   yes  kernel

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>