[LIB]: Introduce struct pcounter

This just generalises what was introduced by Eric Dumazet for the struct proto
inuse field in 286ab3d46058840d68e5d7d52e316c1f7e98c59f:

    [NET]: Define infrastructure to keep 'inuse' changes in an efficent SMP/NUMA way.

Please look at the comment in there to see the rationale.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>