ufs_truncate_blocks(): fix the case when size is in the last direct block

The logics when deciding whether we need to do anything with direct blocks
is broken when new size is within the last direct block.  It's better to
find the path to the last byte _not_ to be removed and use that instead
of the path to the beginning of the first block to be freed...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>