md: fix the test for finding spares in raid5_start_reshape.

As spares can be added to the array before the reshape is started,
we need to find and count them when checking there are enough.
The array could have been degraded, so we need to check all devices,
no just those out side of the range of devices in the array before
the reshape.

So instead of checking the index, check the In_sync flag as that
reliably tells if the device is a spare or this purpose.

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