LU-8708 osc: depart grant shrinking from pinger

LU-8708 osc: depart grant shrinking from pinger

* Removing grant shrinking code outside of pinger, use a workqueue
  to handle grant shrinking timer.
* Enable OSC grant shrinking by default.

bugzilla: 19507

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ifb03c907ad285a307d37d707193cfc32998ba2b2
Reviewed-on: https://review.whamcloud.com/23202
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>