md/raid5: fix calculate of 'degraded' when a replacement becomes active.

When a replacement device becomes active, we mark the device that it
replaces as 'faulty' so that it can subsequently get removed.
However 'calc_degraded' only pays attention to the primary device, not
the replacement, so the array appears to become degraded, which is
wrong.

So teach 'calc_degraded' to consider any replacement if a primary
device is faulty.

This is suitable for -stable as an incorrect 'degraded' value can
confuse md and could lead to data corruption.
This is only relevant for 3.3 and later.

Cc: stable@vger.kernel.org
Reported-by: Robin Hill <robin@robinhill.me.uk>
Reported-by: John Drescher <drescherjm@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -393,6 +393,8 @@ static int calc_degraded(struct r5conf *conf)
 	degraded = 0;
 	for (i = 0; i < conf->previous_raid_disks; i++) {
 		struct md_rdev *rdev = rcu_dereference(conf->disks[i].rdev);
+		if (rdev && test_bit(Faulty, &rdev->flags))
+			rdev = rcu_dereference(conf->disks[i].replacement);
 		if (!rdev || test_bit(Faulty, &rdev->flags))
 			degraded++;
 		else if (test_bit(In_sync, &rdev->flags))
@@ -417,6 +419,8 @@ static int calc_degraded(struct r5conf *conf)
 	degraded2 = 0;
 	for (i = 0; i < conf->raid_disks; i++) {
 		struct md_rdev *rdev = rcu_dereference(conf->disks[i].rdev);
+		if (rdev && test_bit(Faulty, &rdev->flags))
+			rdev = rcu_dereference(conf->disks[i].replacement);
 		if (!rdev || test_bit(Faulty, &rdev->flags))
 			degraded2++;
 		else if (test_bit(In_sync, &rdev->flags))