md/raid5: Fix possible confusion when multiple write errors occur.

commit 5d8c71f
    md: raid5 crash during degradation

Fixed a crash in an overly simplistic way which could leave
R5_WriteError or R5_MadeGood set in the stripe cache for devices
for which it is no longer relevant.
When those devices are removed and spares added the flags are still
set and can cause incorrect behaviour.

commit 14a75d3
    md/raid5: preferentially read from replacement device if possible.

Fixed the same bug if a more effective way, so we can now revert
the original commit.

Reported-and-tested-by: Alexander Lyakas <alex.bolshoy@gmail.com>
Cc: stable@vger.kernel.org (3.2+ - 3.2 will need a different fix though)
Fixes: 5d8c71f
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -3608,7 +3608,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
 			 */
 			set_bit(R5_Insync, &dev->flags);
 
-		if (rdev && test_bit(R5_WriteError, &dev->flags)) {
+		if (test_bit(R5_WriteError, &dev->flags)) {
 			/* This flag does not apply to '.replacement'
 			 * only to .rdev, so make sure to check that*/
 			struct md_rdev *rdev2 = rcu_dereference(
@@ -3621,7 +3621,7 @@ static void analyse_stripe(struct stripe_head *sh, struct stripe_head_state *s)
 			} else
 				clear_bit(R5_WriteError, &dev->flags);
 		}
-		if (rdev && test_bit(R5_MadeGood, &dev->flags)) {
+		if (test_bit(R5_MadeGood, &dev->flags)) {
 			/* This flag does not apply to '.replacement'
 			 * only to .rdev, so make sure to check that*/
 			struct md_rdev *rdev2 = rcu_dereference(