Write a pattern to the last cluster, extend the image by 1 claster, repair
and check that the last cluster still has the same pattern.

Signed-off-by: Alexander Ivanov <alexander.ivanov@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Denis V. Lunev <den@openvz.org>