md: wakeup thread upon rdev_dec_pending()

After each call to rdev_dec_pending() we should wakeup the
md thread if the device is found to be faulty.
Otherwise we'll incur heavy delays on failing devices.

Signed-off-by: Neil Brown <nfbrown@suse.de>
Signed-off-by: Hannes Reinecke <hare@suse.de>