md: don't retry recovery of raid1 that fails due to error on source drive.

If a raid1 has only one working drive and it has a sector which
gives an error on read, then an attempt to recover onto a spare will
fail, but as the single remaining drive is not removed from the
array, the recovery will be immediately re-attempted, resulting
in an infinite recovery loop.

So detect this situation and don't retry recovery once an error
on the lone remaining drive is detected.

Allow recovery to be retried once every time a spare is added
in case the problem wasn't actually a media error.

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

include/linux/raid/md_k.h

@@ -218,6 +218,9 @@ struct mddev_s
 #define	MD_RECOVERY_FROZEN	9
 
 	unsigned long			recovery;
+	int				recovery_disabled; /* if we detect that recovery
+							    * will always fail, set this
+							    * so we don't loop trying */
 
 	int				in_sync;	/* know to not need resync */
 	struct mutex			reconfig_mutex;