md: add support for raising dm events.

dm uses scheduled work to raise events to user-space.
So allow md device to have work_structs and schedule them on an error.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.h

@@ -318,6 +318,7 @@ struct mddev_s
 	struct bio *barrier;
 	atomic_t flush_pending;
 	struct work_struct barrier_work;
+	struct work_struct event_work;	/* used by dm to report failure event */
 };