osd: Correct truncate logic for new EC

osd: Correct truncate logic for new EC

The clone logic in the truncate was only cloning from the truncate
to the end of the pre-truncate object. If the next shard was being
truncated to a shorter length (which is common), then this shard
has a larger clone.

The rollback, however, can only be given a single range, so it was
given a range which covers all clones.  The problem is that if shard
0 is rolled back, then some empty space from the clone was copied
to shard 0.

Fix is easy - calculate the full clone length and apply to all shards, so it matches the rollback.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>
(cherry picked from commit 5d7588c051b31098c9970877ab6a784967ff94c8)