exofs: Fix truncate for the raid-groups case

In the general raid-group case the truncate was wrong in that
it did not also fix the object length of the neighboring groups.

There are two bad cases in the old code:
1. Space that should be freed was not.
2. If a file That was big is truncated small, then made bigger
   again, the holes would not contain zeros but could expose old data.
   (If the growing of the file expands to more than a full
    groups cycle + group size (> S + T))

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>