- helper lov_stripe_size() calculates apparent file size from each stripe

- helper lov_stripe_size() calculates apparent file size from each stripe
- helper lov_merge_attrs() uses lov_stripe_size() and also merges other attrs
- lov_getattr() and lov_open() use lov_merge_attrs() to return size+blocks
- fix lov_stripe_offset() to work properly for truncate up operations
- don't allocate obdos on the stack if we can avoid it
- lov_setattr() is not being used - add LBUG() to ensure we notice when/if
  it does start being used and do a code audit for correctness