net: af_unix should update its inuse counter

This patch is a preparation to namespace conversion of /proc/net/protocols

In order to have relevant information for UNIX protocol, we should use
sock_prot_inuse_add() to update a (percpu and pernamespace) counter of
inuse sockets.

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