staging: lustre: 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.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>