orifs uses a lot of memory after performing a lot of reads

[ezyang]

How to reproduce:

Create an Ori folder, and populate it with some number of files. In my case, my folder had 2G worth of data over 5k files. Now, use 'cp -R' to copy the data outside of the Ori filesystem.

Expected behavior: Ori memory usage increases by a constant amount.

Actual behavior: Ori memory usage increases greatly, and stays that way until the filesystem is unmounted.