writeback: use older_than_this_is_set instead of magic older_than_this == 0

writeback: use older_than_this_is_set instead of magic older_than_this == 0

Currently we use 0 as a special value of work->older_than_this to
indicate that wb_writeback() should set work->older_that_this to current
time. This works but it is a bit magic. So use a special flag in
work_struct for that.

Also fixup writeback from workqueue rescuer to include all inodes.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>