[SOCK]: Enumerate struct proto-s to facilitate percpu inuse accounting (v2).

The inuse counters are going to become a per-cpu array.  Introduce an
index for this array on the struct proto.

To handle the case of proto register-unregister-register loop the
bitmap is used. All its bits manipulations are protected with
proto_list_lock and a sanity check for the bitmap being exhausted is
also added.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>