fix O(num_inodes) behavior of `getStatInfo()`
fix O(num_inodes) behavior of `getStatInfo()`

Summary:
Fix the `getStatInfo()` code to avoid walking all loaded inodes.  That
behavior was okay early on when we were first developing EdenFS and this
API wasn't used in many places.  However, `eden rage` and `eden stats` both
exercise this code path, and it seems pretty bad to walk all loaded inodes in
order to return some stats.

I removed the counts about number of materialized inodes for now, since I
don't think we're really using this metric much.

Reviewed By: chadaustin

Differential Revision: D17578306

fbshipit-source-id: 55ab0209745869b160e91167d6cff7d95f39a95a

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