net: af_netlink should update its inuse counter

In order to have relevant information for NETLINK protocol, in
/proc/net/protocols, 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>