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

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

commit e68a65a47cd5189ced78d39caae585dd72359387
    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 fad5aa9c2cb2bf2a27f01ab67b4146e7c7480ab5
    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: e68a65a47cd5189ced78d39caae585dd72359387
Signed-off-by: NeilBrown <neilb@suse.de>