LU-4423 ptlrpc: use workqueue for pinger

LU-4423 ptlrpc: use workqueue for pinger

lustre has a "pinger" kthread which periodically pings peers to ensure
all hosts are functioning.

This can more easily be done using a work queue.

The SVC_EVENT functionality to wake up the thread can be replaced with
mod_delayed_work().

Change-Id: I6064e9baea186323ab2eb0cca61ed23fcf8b55f7
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-on: https://review.whamcloud.com/31728
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>