This uses work_on_cpu(), rather than altering the cpumask of the
thread which we happen to be.

Note the cleanups:

1) I've removed the CONFIG_NUMA test, since dev_to_node() returns -1
   for !CONFIG_NUMA anyway and the compiler will eliminate it.

2) No need to reset mempolicy to default (a bad idea anyway) since
   work_on_cpu is run from a workqueue.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>