md: Fix is_mddev_idle test (again).

There are two problems with is_mddev_idle.

1/ sync_io is 'atomic_t' and hence 'int'.  curr_events and all the
   rest are 'long'.
   So if sync_io were to wrap on a 64bit host, the value of
   curr_events would go very negative suddenly, and take a very
   long time to return to positive.

   So do all calculations as 'int'.  That gives us plenty of precision
   for what we need.

2/ To initialise rdev->last_events we simply call is_mddev_idle, on
   the assumption that it will make sure that last_events is in a
   suitable range.  It used to do this, but now it does not.
   So now we need to be more explicit about initialisation.

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

include/linux/raid/md_k.h

@@ -51,7 +51,7 @@ struct mdk_rdev_s
 
 	sector_t size;			/* Device size (in blocks) */
 	mddev_t *mddev;			/* RAID array if running */
-	long last_events;		/* IO event timestamp */
+	int last_events;		/* IO event timestamp */
 
 	struct block_device *bdev;	/* block device handle */