Impact: don't play with current's cpumask

Straightforward indirection through work_on_cpu().  One change is
that the error code from microcode_update_cpu() is now actually
plumbed back to microcode_init_cpu(), so now we printk if it fails
on cpu hotplug.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: Peter Oruba <peter.oruba@amd.com>
LKML-Reference: <200903111632.37279.rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>