MDS: support cache eviction
It would be really useful when writing certain kinds of tests (eg, for scrubbing) to be able to know that a particular dentry definitely is or is not in the cache. It's fairly easy to get things in the cache, but almost impossible to know you've gotten them out.
Implement code that will revoke any client capabilities on a dentry (or, for a folder, any dentries in its hierarchy!) and then dump it out of the MDS cache.