`slot_testv_and_readv_and_writev` can cause denial-of-service without payment.

Created by: tomprince

We charge users for mutable files based on the logical size of sharefiles. However, the files we actually use on disk can never shrink. In addition, users can cause those files to grow arbitrarily large without ever paying the cost for that size, by truncating the sharefile in the same call; the user is only charged for the truncated size.