ocfs2_dlmfs: Don't honor truncate. The size of a dlmfs file is LVB_LEN

We want folks using dlmfs to be able to use the LVB in places other than
just write(2)/read(2).  By ignoring truncate requests, we allow 'echo
"contents" > /dlm/space/lockname' to work.

Signed-off-by: Joel Becker <joel.becker@oracle.com>