md/raid10: fix bug which causes all RAID10 reshapes to move no data.

commit 1376512065b23f39d5f9a160948f313397dde972 upstream.

The recent comment:
commit 7e83ccbe
    md/raid10: Allow skipping recovery when clean arrays are assembled

Causes raid10 to skip a recovery in certain cases where it is safe to
do so.  Unfortunately it also causes a reshape to be skipped which is
never safe.  The result is that an attempt to reshape a RAID10 will
appear to complete instantly, but no data will have been moves so the
array will now contain garbage.
(If nothing is written, you can recovery by simple performing the
reverse reshape which will also complete instantly).

Bug was introduced in 3.10, so this is suitable for 3.10-stable.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: Martin Wilck <mwilck@arcor.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>