libext2fs: when truncating file, punch out blocks at end

libext2fs: when truncating file, punch out blocks at end

Currently, ext2fs_file_set_size2 punches out data blocks between the
end of the file and infinity when truncate_block <= old_truncate
(i.e. when you've made the file longer).  This is not a useful
behavior, particularly since it *fails* to punch out the data blocks
when the file is shortened (i.e. truncate_block < old_truncate).  This
seems to be the result of the test being backwards, so fix the code to
punch only when the file is getting shorter.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>