Don't call writeback_set_ratelimit() too often during hotplug

The CPU hotplug callback related to writeback calls writeback_set_ratelimit() during every state
change in the hotplug sequence. This is unnecessary since num_online_cpus() changes only once
during the entire hotplug operation.

So invoke the function only once per hotplug, thereby avoiding the unnecessary repetition
of those costly calculations.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>