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

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: [email protected]
Reported-by: Robin Hill <[email protected]>
Reported-by: John Drescher <[email protected]>
Signed-off-by: NeilBrown <[email protected]>