Revert "vfs: kill write_super and sync_supers"

This reverts commit 8eace0d6f61cb0ce6d5a7cf4a39a40990aeb5f1d.
Revert "vfs: kill write_super and sync_supers"

This reverts commit 27c9b790fac735daf2fcaacc068db27a8c1f6e5d.
CPU hotplug, writeback: 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>