padata: Initialize the padata queues only for the used cpus

padata_alloc_pd set up queues for all possible cpus.
This patch changes this to set up the queues just for
the used cpus.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>