Additional checks to detect when writing corrupted metadata

chenhao.qu and alexander.gorrod I'm opening this ticket to brainstorm what could have gone wrong with the HELP ticket we had and what, if anything, we can or should do to avoid it in the future.

I think detection could be possible if somehow the dhandle->meta_base got corrupted. It does not solve the problem of how that could happen. But it could avoid writing the bad result.