LU-4423 ptlrpc: use delayed_work in sec_gc

LU-4423 ptlrpc: use delayed_work in sec_gc

The garbage collection for security contexts currently has a dedicated
kthread which wakes up every 30 minutes to discard old garbage.

Replace this with a simple delayed_work item on the system work queue.

Change-Id: I5cdb023783104b5e21f4139731065946ed162af1
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Reviewed-on: https://review.whamcloud.com/31724
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>