[LogFS] Erase new journal segments

If the device contains on old logfs image and the journal is moved to
segment that have never been used by the current logfs and not all
journal segments are erased before the next mount, the old content can
confuse mount code.  To prevent this, always erase the new journal
segments.

Signed-off-by: Joern Engel <joern@logfs.org>