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>
diff --git a/drivers/md/md.c b/drivers/md/md.c
index 012482a..f877569 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -6068,6 +6068,8 @@
 	set_bit(MD_RECOVERY_INTR, &mddev->recovery);
 	set_bit(MD_RECOVERY_NEEDED, &mddev->recovery);
 	md_wakeup_thread(mddev->thread);
+	if (mddev->event_work.func)
+		schedule_work(&mddev->event_work);
 	md_new_event_inintr(mddev);
 }