padata: Fix cpu hotplug

We don't remove the cpu that went offline from our cpumasks
on cpu hotplug. This got lost somewhere along the line, so
restore it. This fixes a hang of the padata instance on cpu
hotplug.

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